feat(hooks): add global hooks — PreCompact priorities + git/gh confirm modals

Three new machine-wide hooks installed via `make install-hooks`:

- `precompact-priorities.sh` (PreCompact) — injects a priority block into
  the compaction prompt so the generated summary retains information most
  expensive to reconstruct: unanswered questions, root causes with
  file:line, subagent findings as primary evidence, exact numbers/IDs,
  A-vs-B decisions, open TODOs, classified-data handling.

- `git-commit-confirm.py` (PreToolUse/Bash) — gates `git commit` behind a
  confirmation modal showing parsed message, staged files, diff stats,
  author. Parses both HEREDOC and `-m`/`--message` forms.

- `gh-pr-create-confirm.py` (PreToolUse/Bash) — gates `gh pr create`
  behind a modal showing title, base ← head, reviewers, labels,
  assignees, milestone, draft flag, body (HEREDOC or quoted).

Makefile: adds `install-hooks` / `uninstall-hooks` targets and extends
`list` with a Hooks section. Install prints the settings.json snippet
(in `hooks/settings-snippet.json`) to merge into `~/.claude/settings.json`.

Also: `languages/elisp/claude/hooks/validate-el.sh` now emits JSON with
`hookSpecificOutput.additionalContext` on failure (via new `fail_json()`
helper) so Claude sees a structured error in context, in addition to
the existing stderr output and exit 2.

Patterns synthesized clean-room from fcakyon/claude-codex-settings
(Apache-2.0). Each hook is original content.

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