fix(cases): tell an unreachable path from an absent one, scope the stall cap

The bounded path probe answered "absent" both when a path did not exist and
when it simply did not answer, so a stalled linked case 404'd and the Run
button scaffolded a stray local case over it, and two stalled paths anywhere
made every unrelated path read as absent (hooks skipped, statusLine
overridden, the clone warning lost).

- probePath()/probePathKind() are tri-state: present (or directory/file),
  absent (ENOENT/ENOTDIR only) and unknown (timeout, other errors, refusal).
  boundedPathExists() stays as the display-only boolean.
- A stalled path takes only its own mount out of probing (deepest mount
  point from /proc/self/mounts, never /; just the path itself when there is
  no mount table). Unrelated paths keep probing. The process-wide cap is a
  backstop that answers unknown, and a single-path user request can probe
  past it ({ pastCap: true }), still bounded and still recorded as stalled.
  One console.warn when a path first stalls and one when the cap engages.
- GET /api/cases/:name keeps NOT_FOUND for definite absence only. An
  unreachable linked case answers with its registered path and
  unreachable: true; a local one answers OPERATION_FAILED. runClaude and
  runShell create a case only on errorCode NOT_FOUND. The case list keeps an
  unreachable linked case, marked unreachable, instead of dropping it, and
  fix-plan reports an unreadable plan as an error, not "no plan".
- applyWorkspaceHooks and the statusLine helpers skip only a workspace that
  is absent or on the stalled mount; a capacity refusal no longer stops
  hooks being installed elsewhere, and an unreadable settings file never
  lets the exporter override a user's own statusLine.
- The clone flow's repo-settings warning is back on its synchronous check,
  and stripCaseEnvKeys uses pathExistsForWrite.
- POST /api/sessions (workingDir) and POST /api/quick-start (case folder)
  probe with the bounded probe instead of statSync/existsSync. Missing and
  non-directory keep INVALID_INPUT; unknown is OPERATION_FAILED, and
  quick-start never scaffolds over a folder that did not answer.
- PATH_PROBE_TIMEOUT_MS and MAX_STALLED_PATH_PROBES move to
  src/config/path-probe.ts, overridable via CODEMAN_PATH_PROBE_TIMEOUT_MS
  (default 1500) and CODEMAN_PATH_PROBE_MAX_STALLED (default 3), and are
  documented in the Settings Reference.
- The probe is exported from the utils barrel and imported from there.

src/web/public/session-ui.js

@@ -1874,10 +1874,14 @@ Object.assign(CodemanApp.prototype, {
     try {
       // Get case path first
       const caseRes = await fetch(`/api/cases/${caseName}`);
-      let caseData = (await caseRes.json())?.data ?? {};
+      const caseLookup = await caseRes.json();
+      let caseData = caseLookup?.data ?? {};
 
-      // Create the case if it doesn't exist
+      // Create the case only when the server says it does not exist. Any other
+      // failure (a linked folder on a mount that is not answering) must not
+      // scaffold a same-name local case that would then shadow the real one.
       if (!caseData.path) {
+        if (caseLookup?.errorCode !== 'NOT_FOUND') throw new Error(caseLookup?.error || 'Case lookup failed');
         const createCaseRes = await fetch('/api/cases', {
           method: 'POST',
           headers: { 'Content-Type': 'application/json' },
@@ -2084,10 +2088,14 @@ Object.assign(CodemanApp.prototype, {
     try {
       // Get the case path
       const caseRes = await fetch(`/api/cases/${caseName}`);
-      let caseData = (await caseRes.json())?.data ?? {};
+      const caseLookup = await caseRes.json();
+      let caseData = caseLookup?.data ?? {};
 
-      // Create the case if it doesn't exist
+      // Create the case only when the server says it does not exist. Any other
+      // failure (a linked folder on a mount that is not answering) must not
+      // scaffold a same-name local case that would then shadow the real one.
       if (!caseData.path) {
+        if (caseLookup?.errorCode !== 'NOT_FOUND') throw new Error(caseLookup?.error || 'Case lookup failed');
         const createCaseRes = await fetch('/api/cases', {
           method: 'POST',
           headers: { 'Content-Type': 'application/json' },