fix(test): gstack-config tests must win the state-dir precedence race

[tranthanhnhatkhoa]

The bug

browse/test/gstack-config.test.ts drives GSTACK_STATE_DIR — the lowest-priority of the four state-dir sources bin/gstack-config:17 reads:

GSTACK_STATE_ROOT > GSTACK_HOME > GSTACK_STATE_DIR > $HOME/.gstack

The helper spreads ...process.env into the spawn env, so if either higher-priority variable is set in the parent process, it silently outranks the temp dir and the script reads somewhere else. Every case asserting on fixture content then sees an empty config.

Not hypothetical — five suites assign GSTACK_HOME at module scope and never restore it:

browse/test/telemetry.test.ts:11
browse/test/cdp-e2e.test.ts:21
browse/test/domain-skills-storage.test.ts:7
browse/test/domain-skills-e2e.test.ts:20
test/helpers/budget-override.test.ts:18

bun shares one process across files, so whether these pass depends on file ordering:

	result
bun test browse/test/gstack-config.test.ts	26 pass, 0 fail
same file inside bun run test	17 pass, 9 fail

The fix

In the harness, not the leaking suites. A test that spawns a subprocess should fully specify the env it depends on rather than inherit whatever ran first — that makes it immune to any leaker, including ones added later. Patching the five would fix today and regress the first time someone adds a sixth.

GSTACK_STATE_DIR stays the knob, so the legacy alias keeps its coverage, and the per-case extraEnv override at line 106 is unaffected.

Verified

clean env                    26 pass  0 fail
GSTACK_HOME poisoned         26 pass  0 fail   (was 17 pass, 9 fail)
GSTACK_STATE_ROOT poisoned   26 pass  0 fail
full suite                   32 fails -> 23    (all 9 gstack-config gone)

Environment: bun 1.3.14, Ubuntu 24.04.4 LTS, main @ 960c3a8d (v1.60.2.0).

Two things found alongside, not fixed here

The module-scope leaks are still there. This PR makes one harness immune; it doesn't stop the pollution. Anything reading GSTACK_HOME in-process (not via a spawned CLI) is still exposed. Worth a separate pass moving those five into beforeAll/afterAll with restore.

The remaining 23 failures are unrelated to this. 15 of them (gstack-team-init, gstack-session-update) are environmental: those cases git init and commit into temp repos, and fail with Author identity unknown on a machine with no global user.name/user.email. Supplying GIT_AUTHOR_* takes test/team-mode.test.ts from 9 pass / 15 fail to 24 pass / 0 fail. That may be worth a fixture-level identity so the suite doesn't depend on the developer's global git config — happy to send that separately if you want it.

I also hit #2435 while doing this (suite exits 0 with failures) and left a repro there.

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