fix: install/uninstall-hooks merge per event, keep foreign hooks (GIT-120)

[nTEG-dev]

GIT-120: install-hooks and uninstall-hooks keep foreign hooks

Just upgrade the package. Nothing to change on your database. No store involvement.

What was wrong

• install-hooks replaced settings.hooks wholesale, and uninstall-hooks deleted the key. Every hook a user or another tool had configured was lost.
• "Already installed" was detected by any gitmem substring. A foreign hook such as bash ~/code/gitmem-notes/lint.sh made main skip the install entirely.
• An unparseable settings.json was replaced with a fresh file.
• Cursor's install-hooks also replaced hooks wholesale.

Fix (bin/hooks-merge.js, used by bin/gitmem.js)

• Merge per event and per command. Only commands gitmem installed are replaced or removed. A matcher group keeps its other commands. Events install-hooks does not manage stay untouched, including the UserPromptSubmit hook that gitmem init installs.
• gitmem commands are recognised by gitmem's script paths (.gitmem/hooks/*.sh, gitmem-mcp/hooks/scripts/*.sh, legacy gitmem-hooks).
• The file is backed up to <file>.gitmem-backup-<timestamp> before each write. The output lists the change per event and the backup path.
• An unparseable file, or a hooks value of the wrong shape, exits 1 with "Nothing was changed".
• Applies to Claude Code .claude/settings.json and Cursor .cursor/hooks.json.

Evidence

New CLI e2e tests/e2e/git-120-hooks-merge.test.ts runs the real CLI in a scratch repo with foreign hooks. It is wired into CI next to the GIT-89 step.

• Branch: 9/9 pass.
• Main: 6 of the first 8 fail (the 9th, init's auto-retrieve hook, was added afterwards):

× install-hooks keeps foreign hooks, backs up first, and prints what changed   (main: "already installed", 0 gitmem hooks)
× install-hooks --force twice replaces gitmem's hooks without duplicating them
× uninstall-hooks removes only gitmem's commands ...                            (main: hooks undefined)
× uninstall-hooks keeps a foreign command that shares a matcher group
× an unparseable settings.json is not overwritten                              (main: exit 0, file replaced)
× install and uninstall keep a foreign Cursor hook

tsc clean. Unit 1357, unchanged.

Old-behaviour tests changed

tests/e2e/cli-fresh-install.test.ts pinned the wipe:

• "install-hooks --force preserves permissions" now also expects the fixture's foreign echo old hook to survive.
• "uninstall-hooks removes hooks from settings" expected hooks undefined. It now expects only echo old to remain.
• "uninstall-hooks is idempotent" now expects "No gitmem hooks found".

Two other tests in that file fail on main and on this branch alike: "gitmem init creates .gitmem/ with starter scars" and "session-close-check.sh ceremony wording". That file is not in CI yet; wiring it is part of GIT-115.

Not changed, flagged

• bin/init-wizard.js and bin/uninstall.js still match hooks by a bare gitmem substring and drop whole matcher groups. That can remove a foreign command that shares a group. They are left for GIT-115, which reworks the wizard.
• install-hooks installs no UserPromptSubmit hook, but gitmem init does. The two hook sets differ.
• uninstall-hooks still clears /tmp/gitmem-hooks-* machine-wide. That is existing behaviour, and the e2e exercises it.

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