feat(hooks): independent precompact toggle via config + env var

[rusel95]

🚫 The problem

You are in a long session. Context fills up. You type /compact. Nothing happens.

The precompact hook is blocking compaction. You cannot disable it without also disabling the stop hook. You are stuck.

This was the #1 reported issue after the initial precompact hook shipped. PR #863 fixed the hard-block by making precompact always return {} — but that dropped the save nudge entirely. Users now get no signal that compaction is imminent.

✅ What this does

Adds an independent toggle for the precompact hook:

// ~/.mempalace/config.json
{"hooks": {"precompact": false}}

# Or via env var
MEMPALACE_HOOKS_PRECOMPACT=false

When disabled, /compact proceeds immediately — no mine, no delay.

🔑 Why independent controls matter

Scenario	save_interval	precompact
Normal use	15 (default)	true (default)
Quiet mode — fewer interruptions	50	true
Precompact causing issues	15	false
Fully silent	0	true

Disabling save_interval does not disable precompact. Disabling precompact does not disable the stop hook. They are independent by design — so you can fix one without breaking the other.

🧪 Tests

All 104 existing hook and config tests pass.

python -m pytest tests/test_hooks_cli.py tests/test_config.py -q
104 passed

Depends on: #1345 (save_interval config, same config.py property pattern)
Closes #949. Closes #906.

[rusel95]

✅ End-to-end testing completed

Verified against the real MempalaceConfig class and hook_precompact() function.

Tests verified:

• Default precompact = true ✅
• MEMPALACE_HOOKS_PRECOMPACT=false → disabled ✅
• MEMPALACE_HOOKS_PRECOMPACT=true → enabled ✅
• Independence test: save_interval=0 + no precompact env → precompact still true ✅

This is the critical test — it proves the two controls are truly independent. Disabling periodic saves does not silently disable the last-chance save before compaction.

python -m pytest tests/test_hooks_cli.py tests/test_config.py -v
104 passed in 0.59s

Full suite: 0 regressions.