fix(custom-model): root-cause and fix DeepSeek's HTTP_404 (missing /v1)

DeepSeek Harness's own bundled provider module
(@deepseek-ai/dsh-llm-deepseek) builds its request URL as
`${DEEPSEEK_BASE_URL}/chat/completions` with no `/v1` insertion of its
own (its real public API, https://api.deepseek.com, expects the
caller's base URL to already carry any needed prefix), while
llama-swap/llama.cpp only ever serves the OpenAI-conventional
`/v1/chat/completions`.

Confirmed two ways:
- Installed the real @deepseek-ai/dsh package (all its actual
  published dependencies) into a scratch dir purely to read
  dsh-llm-deepseek's source: `fetch(`${connection.baseURL}/chat/
  completions`, ...)`, baseURL read straight from DEEPSEEK_BASE_URL —
  the same grep-the-real-source bar pi/grok's fixes were held to.
- Live against the test-picker's llama-swap: `POST <baseUrl>/chat/
  completions` -> 404, `POST <baseUrl>/v1/chat/completions` -> 200,
  same endpoint. dsh's own error template ("DeepSeek API error (HTTP
  ${status})") reproduces the originally-reported
  "dsh: HTTP_404: DeepSeek API error (HTTP 404)" exactly.

- New registry field `appendV1Suffix` (env kind only, deepseek's entry
  alone — claude/gemini must NOT get it, since claude was already
  confirmed working against the unmodified baseUrl). When set,
  buildCustomModelInjection runs endpoint.baseUrl through the same
  withV1Suffix() helper configDir-kind CLIs (pi/grok/codex) already
  use, instead of writing it verbatim.

Not yet re-run end-to-end through a real dsh binary — no install
available in this environment (not in PATH, and the test-picker
container doesn't bundle it) — so this is source-confirmed and
live-verified at the HTTP level, not yet promoted to "verified"
alongside claude/opencode/pi/grok/omp. Docs (custom-model-endpoints.md,
the plan doc's confidence table, the wiki page, CLAUDE.md) all updated
to reflect this precisely rather than leaving the old "root cause not
identified" claim in place.

2 new/updated tests for the /v1 suffix (including idempotency against
a baseUrl that already ends in /v1) plus a corrected mock-server
contract test. Typecheck/lint clean; full suite shows no new
regressions.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RqZeHrRS6DYcGcGX2p9EwG

docs/wiki/Custom-Model-Endpoints.md

@@ -146,13 +146,13 @@ entry. "Launch anyway" is still there if you want to try regardless.
 
 ## Which harnesses actually work
 
-| Harness                                  | Status                                                                                                                                        |
-| ---------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------- |
-| **Claude Code, opencode, Pi, Grok, OMP** | Verified end-to-end against a real local server.                                                                                              |
-| **Codex**                                | Config is correct, but Codex only speaks the Responses API, which llama.cpp-style servers don't implement. A protocol gap, not a Codeman bug. |
-| **Gemini**                               | Fails with an auth error gemini-cli raises once redirected. Unresolved; don't rely on it yet.                                                 |
-| **DeepSeek**                             | Reaches the server but gets a consistent 404. Root cause not identified.                                                                      |
-| **Antigravity**                          | No known custom-endpoint mechanism at all. Not offered.                                                                                       |
+| Harness                                  | Status                                                                                                                                                                                                               |
+| ---------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
+| **Claude Code, opencode, Pi, Grok, OMP** | Verified end-to-end against a real local server.                                                                                                                                                                     |
+| **Codex**                                | Config is correct, and plain chat can work against a server that speaks the Responses API — but a real tool-call attempt comes back as inert text instead of running, so it's still not usable for real coding work. |
+| **Gemini**                               | Fails with an auth error gemini-cli raises once redirected. Unresolved; don't rely on it yet.                                                                                                                        |
+| **DeepSeek**                             | The original 404 is root-caused and fixed (DeepSeek Harness's own code was missing a `/v1` most local servers require) — not yet re-run against a real `dsh` install to confirm end-to-end.                          |
+| **Antigravity**                          | No known custom-endpoint mechanism at all. Not offered.                                                                                                                                                              |
 
 Which harnesses show up in the Run-menu picker is read live off Codeman's own CLI registry,
 not a fixed list here, so this table can go stale before this page does — a greyed-out or