fix(hooks): refresh nerf statusline on PreCompact + SessionStart:compact

[bakeb7j0]

Summary

Layer 2 of the stale-statusline fix. Wires mcp-server-nerf v1.2.3's new clear-indicator and refresh-indicator subcommands into Claude Code's hook system so the nerf widget clears before compaction and repaints after the compact resume — fixing the ~126% pre-compact percentage that lingered until a manual /nerf status.

Changes

• scripts/hooks/nerf/pre-compact.sh (new) — CC PreCompact hook. Reads stdin JSON, shells out to ~/.local/bin/nerf-server clear-indicator (with --session-id when present). Best-effort: always exits 0; logs to stderr on subprocess failure per lesson_best_effort_must_log.md.
• scripts/hooks/nerf/session-start-compact.sh (new) — CC SessionStart hook scoped to matcher: "compact". Same shape, calls refresh-indicator --session-id <id>.
• config/settings.template.json — adds the new PreCompact block (matcher: "*") and a second SessionStart array entry (matcher: "compact") alongside the preserved crystallizer entry.

Both scripts pass shellcheck and run cleanly when ~/.local/bin/nerf-server is missing/older (graceful no-op via [[ -x "$NERF_BIN" ]] guard plus best-effort exit 0).

Linked Issues

Closes #555

Dependencies

• mcp-server-nerf v1.2.3 (release, PR #27, issue #26) — installs to ~/.local/bin/nerf-server. Older binaries don't recognize the subcommands; the hooks log and exit 0 in that case (no breakage).

Upgrade gap (acknowledged, follow-up #556)

merge_settings() in install only adds new event keys from the template — it does not union-merge new matcher entries into an existing event array. So a user who already has SessionStart in their ~/.claude/settings.json will get PreCompact (genuinely new) but will NOT get the second SessionStart: matcher "compact" entry. Workaround: fresh install (mv ~/.claude/settings.json{,.bak} && ./install) or manually copy the new SessionStart entry from config/settings.template.json.

The complete fix to the merge logic is tracked in #556 so this PR doesn't expand scope.

Test Plan

• ./scripts/ci/validate.sh → 122 passed, 0 failed (shellcheck + shfmt + regression suite)
• ./install --check → confirms new hook scripts deploy to ~/.local/bin/hooks/nerf/ and PreCompact is recognized as a missing event key
• Manual smoke: echo '{"session_id":"<uuid>",...}' | scripts/hooks/nerf/pre-compact.sh → exit 0, no stderr noise; same for session-start-compact.sh against a freshly-installed v1.2.3 binary
• trivy fs --scanners vuln --severity HIGH,CRITICAL . → 0 findings
• Code review: 1 Important fixed (misleading "falling back to resolver" log string in session-start-compact.sh); 1 Important deferred to fix(install): union-merge hook matcher arrays in merge_settings() #556 (install merge gap, predates this PR)
• End-to-end: trigger a compact in a live Claude Code session post-merge, confirm indicator clears during PreCompact and repaints with post-compact size after SessionStart:compact (requires this PR merged + a fresh-install or manual settings.json edit per upgrade-gap caveat above)