feat(cases): clone a Git repository as a new case (#236)

Adds an Add Case -> "Clone Repo" tab plus two endpoints, implementing
@DodgyBadger's proposal in #236: clone a public repository straight into
codeman-cases/<name> and register it as a normal local case.

POST /api/cases/clone is synchronous by design (request held open, bounded
by GIT_CLONE_TIMEOUT_MS): no job store, no polling, no cancellation
surface. Success broadcasts the usual case:created event, so the case
still appears when a proxy idle-timeout kills the request mid-clone.

POST /api/cases/clone-preflight runs `git ls-remote --symref` so the UI can
say, while the user is still typing, whether the URL is cloneable without
credentials, what its default branch is, and which branches/tags exist.

Core lives in src/git-clone.ts, split into a pure half (URL parse, argv/env,
ls-remote parse, stderr classification) and a thin IO half, so every
security decision is unit-testable without spawning anything:

- `<name>::<payload>` transports are refused as a family, not by name:
  ext:: is the famous one, but any of them dispatches to git-remote-<name>
  and turns a clone into arbitrary command execution.
- A leading `-` is refused AND every spawn puts `--` before the operands.
  Either alone is one edit away from being a hole.
- argv arrays, never a shell. URLs carrying user:password@ are refused.
- gitNonInteractiveEnv() closes all four ways git can block on a prompt
  with no terminal attached (terminal prompt, askpass/GUI, ssh, GCM).
  HOME/PATH stay inherited, so a user's own credential helper or ssh agent
  keeps working; Codeman itself collects and stores nothing.
- The timeout signals the process GROUP, since clone fans out into
  git-remote-https/index-pack children that outlive a signal to the parent.
- Bounded output (redacted stderr tail, capped ls-remote stdout, 500 refs
  each) and a global 2-op pool, so N large clones cannot exhaust the host.

Repository contents beat scaffolding: an existing CLAUDE.md is kept, hooks
are merged into whatever .claude/settings.local.json the repo shipped, and
a repo that ships its own Claude settings is reported back as a warning
(those hooks run locally as soon as a session starts there). A failed clone
removes only the directory the attempt created, and refuses a pre-existing
destination outright, so it can never squat on a case name.

Not admin-gated in multi-user mode, unlike /api/cases/link: it writes only
inside the caller's own case space. Local-path/file:// sources are the
exception and stay admin-only there.

UI: live verdict under the URL field, case name filled from the parsed repo
until the user types their own, branch/tag as a datalist of the remote's
real refs, optional shallow clone, and a Brain picker (installed CLIs only)
that points the Run button at the chosen agent. Starting a session stays
opt-in. The tab hides itself when the server reports no git.

Tests: the pure half exhaustively (every refusal has a case), plus real git
against a real local bare repo for clone/ref/timeout/cleanup, and a
route-level suite with unmocked fs that clones through the endpoint.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/web/public/index.html

@@ -2043,6 +2043,7 @@
         </div>
         <div class="modal-tabs">
           <button class="modal-tab-btn active" data-tab="case-create">Create New</button>
+          <button class="modal-tab-btn" data-tab="case-clone" id="caseCloneTabBtn">Clone Repo</button>
           <button class="modal-tab-btn" data-tab="case-link">Link Existing</button>
           <button class="modal-tab-btn" data-tab="case-remote">Remote</button>
           <button class="modal-tab-btn" data-tab="case-docker">Docker</button>
@@ -2108,6 +2109,51 @@
               </div>
             </details>
           </div>
+          <!-- Clone Repo Tab (issue #236) -->
+          <div class="modal-tab-content hidden" id="case-clone">
+            <div class="form-row">
+              <label>Repository URL</label>
+              <input type="url" id="cloneRepoUrl" placeholder="https://github.com/owner/repo.git" autocomplete="off" autocapitalize="off" autocorrect="off" spellcheck="false" oninput="app.onCloneUrlInput()">
+              <span class="form-hint clone-status" id="cloneRepoStatus">Public repositories only: Codeman clones with no credentials.</span>
+            </div>
+            <div class="form-row">
+              <label>Case Name</label>
+              <input type="text" id="cloneCaseName" placeholder="repo" pattern="[a-zA-Z0-9_-]+" autocomplete="off" autocapitalize="off" spellcheck="false" oninput="app.onCloneNameEdited()">
+              <span class="form-hint">Filled in from the URL. Cloned into ~/codeman-cases/&lt;name&gt;, so deleting the case deletes this working tree.</span>
+            </div>
+            <div class="form-row">
+              <label>Branch or Tag (optional)</label>
+              <input type="text" id="cloneRepoRef" list="cloneRepoRefOptions" placeholder="default branch" autocomplete="off" autocapitalize="off" spellcheck="false">
+              <datalist id="cloneRepoRefOptions"></datalist>
+              <span class="form-hint" id="cloneRefHint">Leave blank for the repository's default branch.</span>
+            </div>
+            <div class="form-row">
+              <label>Brain</label>
+              <select id="cloneCaseBrain" class="form-select">
+                <option value="">Leave the Run button as it is</option>
+                <option value="claude" data-cli="claude">Claude Code</option>
+                <option value="codex" data-cli="codex">Codex</option>
+                <option value="gemini" data-cli="gemini">Gemini</option>
+                <option value="opencode" data-cli="opencode">OpenCode</option>
+                <option value="antigravity" data-cli="antigravity">Antigravity</option>
+                <option value="shell">Shell (no agent)</option>
+              </select>
+              <span class="form-hint">Which CLI to point the Run button at once the clone finishes. Changeable any time from the Run dropdown.</span>
+            </div>
+            <details class="advanced-options">
+              <summary>Clone options</summary>
+              <div class="advanced-options-content">
+                <div class="form-row">
+                  <label class="checkbox-row"><input type="checkbox" id="cloneShallow"> Shallow clone (--depth 1)</label>
+                  <span class="form-hint">Much faster on big repositories, but there is no history to read afterwards.</span>
+                </div>
+                <div class="form-row">
+                  <label class="checkbox-row"><input type="checkbox" id="cloneStartSession"> Start a session when the clone finishes</label>
+                </div>
+              </div>
+            </details>
+            <span class="form-hint" id="cloneCaseNote" style="margin-top: 8px; display: block;">The clone runs while this request is open, so a large repository takes a while. The case appears as soon as git finishes, even if the browser gave up waiting.</span>
+          </div>
           <!-- Link Existing Tab -->
           <div class="modal-tab-content hidden" id="case-link">
             <div class="form-row">