v1.15.1.0 fix(gstack-slug): subdir guard + .gstack-slug override + --reset (#1125)

[gregario]

Summary

Fixes #1125 — gstack-slug inherits the outer repo's slug when \$PWD is a subdirectory of a different git repo, then locks the wrong slug into ~/.gstack/slug-cache/ with no recovery path.

Before

$ cd /Users/me/workspace                  # outer repo, origin = me/workspace
$ mkdir IoTopia && cd IoTopia              # standalone project, no .git
$ gstack-slug
SLUG=me-workspace                          # ← wrong: inherited from parent

~/.gstack/projects/me-workspace/ now collects artifacts for an unrelated project. Manual cleanup of both ~/.gstack/projects/ and ~/.gstack/slug-cache/ was needed to recover.

After

$ cd /Users/me/workspace/IoTopia
$ gstack-slug
SLUG=IoTopia                               # ← falls through to basename

# Or, set a per-project override:
$ echo my-real-project > .gstack-slug
$ gstack-slug --reset && gstack-slug
SLUG=my-real-project

Three changes

1. Subdir guard. Compare git rev-parse --show-toplevel against \$PWD. If they differ, skip the remote-URL inference and fall through to .gstack-slug or basename. Eliminates the silent cross-contamination.
2. .gstack-slug per-project override. A one-line file in \$PWD (sanitized to [a-zA-Z0-9._-]) wins over git inference. Cheap, explicit, survives gstack-upgrade.
3. gstack-slug --reset. Removes the cache entry for the current PWD without manual cleanup of ~/.gstack/slug-cache/.

Mirrors the shape of @snowmaker's merged PR #897 (deterministic slugs across sessions): one bin/ script change, one regression test file, zero behavior change for the happy path.

Test plan

• bun test test/gstack-slug.test.ts — 10 cases pass
  • Subdir guard regression (the reproducer from the issue)
  • PWD at toplevel still inherits remote slug (happy path unchanged)
  • No git repo at all falls back to basename (happy path unchanged)
  • .gstack-slug override at toplevel and in subdir
  • Override input sanitization (rejects ;, spaces, multi-line)
  • --reset clears the cache file; no-op when cache absent
  • Cache stability: cached slug persists when remote disappears
• bun test (full free suite) — green
• No changes to existing happy-path behavior

Files

• bin/gstack-slug — fix
• test/gstack-slug.test.ts — new, 10 cases
• VERSION, CHANGELOG.md — release v1.15.1.0

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