fix: use #!/usr/bin/env bash in community .sh hooks for distro portability

The three opt-in bash hooks (gsd-phase-boundary.sh, gsd-session-state.sh,
gsd-validate-commit.sh) shipped with #!/bin/bash, which fails on distros
that don't ship bash at /bin/bash (NixOS, minimal Alpine images, some
container runtimes). POSIX guarantees /bin/sh but not /bin/bash.

This is latent in the default install path because Claude Code wires the
hooks as `bash <path>` from settings.json (PATH-resolved — the script's
own shebang is read as a comment by bash). The fix matters when scripts
are run directly: tests, future installer changes, or manual debugging.

Changes:
- hooks/gsd-{phase-boundary,session-state,validate-commit}.sh: shebang
  switched to #!/usr/bin/env bash, matching the convention already used
  in scripts/*.sh.
- tests/bug-2136-sh-hook-version.test.cjs: assertion updated to expect
  the new shebang; comment updated to spell out the rationale.
- tests/bug-2979-hook-absolute-node.test.cjs: doc-comment updated — the
  prior wording cited "POSIX std PATH always has /bin" as the reason
  bare `bash` is OK. The actual reason is that bare `bash` is
  PATH-resolved, which is portable across distros that don't ship
  /bin/bash. POSIX std PATH guarantees /bin/sh, not /bin/bash.
- bin/install.js::buildHookCommand: comment block clarifying the same.
  No behavior change in this file — bare `bash` was already correct.
- .changeset/portable-bash-shebang-hooks.md: changeset entry.

Verified locally on NixOS:
- npm run build:hooks: hooks/dist/*.sh shebangs propagate correctly.
- node --test tests/bug-2136-*.cjs tests/bug-2979-*.cjs
  tests/bug-1817-*.cjs tests/bug-1834-*.cjs tests/bug-1906-*.cjs
  tests/bug-2557-*.cjs tests/bug-3017-*.cjs tests/security-scan.test.cjs
  tests/hooks-doc-parity.test.cjs: 126/126 pass.
- node scripts/run-tests.cjs (full suite): 6944 pass / 0 fail / 5 skip.

tests/bug-2979-hook-absolute-node.test.cjs

@@ -20,7 +20,8 @@ process.env.GSD_TEST_MODE = '1';
  * resolveNodeRunner helper, asserting on structured records:
  *  - the runner field is an absolute path (not bare 'node')
  *  - it ends with /node or \\node (or .exe on Windows simulation)
- *  - .sh hooks still use bare 'bash' (POSIX std PATH always has /bin)
+ *  - .sh hooks still use bare 'bash' (PATH-resolved; portable across
+ *    distros that don't ship /bin/bash, like NixOS)
  *
  * No source-grep on install.js content — assertions go against the
  * value returned by the exported function and the parsed structure of