docs(test): name the real reason the suite could reach ~/.codeman

#356 stopped a bare suite run from overwriting the production
`remote-hosts.json` by pointing `CODEMAN_DATA_DIR` at a throwaway dir, and it
gated every case-tree delete on the temp HOME. Both changes are right; the
explanation written next to them is not. It says `os.homedir()` reads
/etc/passwd rather than `$HOME` on Linux, which would mean the temp HOME in
test/setup.ts never worked. It does: libuv checks the env var before the passwd
entry (measured: `HOME=/tmp/x node -e 'console.log(os.homedir())'` prints
/tmp/x), and CLAUDE.md's testing section relies on exactly that.

What bypasses the temp HOME is `CODEMAN_DATA_DIR` itself. `getDataDir()` reads
it as an absolute override before it looks at `homedir()`, so one inherited from
the shell (a second instance, a beta run) sends the whole suite at the real data
dir. That is the case setup.ts now closes, and #371 names the same variable from
the other direction.

The comments in setup.ts, the `safeRmHomeTree` helper, the voice-routes and
case-clone tests now say that, and the containment gate is described as what it
is: defense in depth. CLAUDE.md's testing paragraph gets the same note so the
next reader does not chase a homedir() bug that does not exist.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Qg6bcATm1pNNY4kQWGwzgu

test/routes/case-clone-routes.test.ts

@@ -9,10 +9,10 @@
  * working tree in the case directory, that scaffolding does not overwrite the
  * repository's own files, and that a rejected URL never reaches git.
  *
- * `test/setup.ts` points HOME at a per-file temp dir, but CASES_DIR is
- * `join(homedir(), 'codeman-cases')` and `os.homedir()` ignores the HOME
- * override on some platforms/Node builds — so cleanup below goes through
- * `safeRmHomeTree`, which refuses to delete anything outside the temp HOME.
+ * `test/setup.ts` points HOME at a per-file temp dir, so CASES_DIR
+ * (`join(homedir(), 'codeman-cases')`) resolves inside the fixture; cleanup
+ * below still goes through `safeRmHomeTree`, which refuses to delete anything
+ * outside the temp HOME, so a wrong anchor can never reach the real tree.
  *
  * Port: N/A (app.inject).
  */

test/routes/voice-routes.test.ts

@@ -25,9 +25,10 @@ import { registerVoiceRoutes, _resetVoiceStreamCountForTesting } from '../../src
 import { MAX_CONCURRENT_STREAMS } from '../../src/config/voice.js';
 
 // SAFETY (2026-08-29): anchor on the REDIRECTED test HOME (process.env.HOME,
-// which test/setup.ts points at a throwaway dir) instead of os.homedir().
-// On some Linux builds os.homedir() reads /etc/passwd and would resolve to the
-// REAL home, clobbering the user's ~/.claude/.credentials.json.
+// which test/setup.ts points at a throwaway dir). `os.homedir()` follows it too,
+// but this file writes and deletes `~/.claude/.credentials.json`, the one file
+// where a wrong anchor would sign the developer out of their own CLI, so it
+// fails loudly if setup.ts did not run rather than trusting any fallback.
 function testHome(): string {
   if (!process.env.HOME) throw new Error('process.env.HOME unset — test/setup.ts must run first');
   return process.env.HOME;