feat(hooks): post_compact event with realized compaction outcome

[emal-avala]

Summary

Pairs with `PreCompact` (which fires before compaction with an estimated tokens-freed number). `PostCompact` fires after compaction finishes, carrying the ground-truth delta: `messages_before`, `messages_after`, and the actual `freed_tokens`.

Audit hooks can now record both the forecast and the reality. That matters because the three compaction paths — microcompact, LLM compaction, and the context-collapse fallback — don't always free what the estimate predicted. LLM compaction can be blocked by a cancel; the collapse fallback frees a different shape of context; and in the no-op case nothing changes at all.

Wiring

• New `HookEvent::PostCompact` variant with snake_case serde
• `fire_post_compact_hooks(messages_before, messages_after, freed_tokens)` on `QueryEngine`
• `/compact` handler fires it after microcompact, including on no-op (`freed=0`), so user-invoked `/compact` always reaches hooks
• `run_turn_with_sink` fires it once at the end of the auto-compact block with the cumulative delta across all paths taken; skipped when nothing actually changed so hooks aren't spammed every turn
• `HOOK_EVENT_CATALOG`, `format_hook_event`, `parse_hook_event`, and the module-header docs all updated

Test plan

• `cargo clippy --workspace --tests --no-deps -- -D warnings` — clean
• `cargo test -p agent-code-lib --lib hooks::` — 5 pass (4 prior + 1 new)
• `cargo test -p agent-code-lib --lib config::` — 69 pass (includes new serde round-trip)
• `cargo test -p agent-code --bin agent commands::tests::parse_hook_event` — 6 pass (5 prior + 1 new)
• `format_hook_event_covers_every_variant` now asserts `PostCompact` is mapped
• `parse_hook_event_covers_every_variant_in_catalog` keeps catalog and parser in lock-step