feat: configurable auto-save interval with independent precompact control

[rusel95]

Closes #494

Summary

Replaces the hardcoded SAVE_INTERVAL = 15 with a user-configurable hooks.save_interval and adds independent hooks.precompact control. This is an alternative to #711 that provides granular frequency control instead of a binary on/off toggle.

Why this is better than #711

	#711 (binary toggle)	This PR (interval + independent precompact)
Stop hook control	auto_save: true/false — all or nothing	save_interval: 30 — any frequency (0 = off)
Precompact	Disabled together with stop hook — data loss risk	Independent toggle — precompact stays on by default
Config duplication	Same check in 2 bash scripts + Python (3 places)	Python only — bash scripts are thin wrappers
User flexibility	"Too noisy? Turn it off entirely"	"Too noisy? Set interval to 50 instead of 15"

Config

// ~/.mempalace/config.json
{
  "hooks": {
    "save_interval": 30,
    "precompact": true
  }
}

Setting	Default	Env var	Effect
save_interval	15	MEMPALACE_HOOKS_SAVE_INTERVAL	Messages between auto-saves. 0 = disabled
precompact	true	MEMPALACE_HOOKS_PRECOMPACT	Block before context compaction. Last chance to save

Key design decision: precompact defaults to true and is controlled separately. Setting save_interval: 0 disables periodic saves but precompact still fires before context is lost. This prevents the data loss scenario where #711's auto_save: false silently disables both hooks.

Changes

File	What
mempalace/config.py	hooks_save_interval and hooks_precompact properties with env var overrides
mempalace/hooks_cli.py	Reads interval from config instead of hardcoded constant; precompact checks its own toggle; shortened block reasons naming exact MCP tools
tests/test_config.py	11 new tests: interval default/config/env/zero/negative, precompact default/disabled/env, independence
tests/test_hooks_cli.py	7 new tests: stop disabled by zero interval, custom intervals (5, 50), precompact disabled/enabled, stop+precompact independence

Test plan

• test_hooks_save_interval_default — default is 15
• test_hooks_save_interval_from_config — reads from config.json
• test_hooks_save_interval_zero_disables — 0 disables stop hook
• test_hooks_save_interval_env_override — env var overrides config
• test_hooks_save_interval_env_zero — env var can set to 0
• test_hooks_save_interval_negative_clamped — negative clamped to 0
• test_hooks_precompact_default — default is true
• test_hooks_precompact_disabled — can be disabled via config
• test_hooks_precompact_env_override — env var overrides
• test_hooks_save_interval_and_precompact_independent — disabling one doesn't affect the other
• test_stop_hook_disabled_by_zero_interval — stop hook passes through
• test_stop_hook_custom_interval — save_interval=5 triggers at 5 messages
• test_stop_hook_custom_interval_not_reached — save_interval=50 doesn't trigger at 15
• test_precompact_disabled_by_config — precompact passes through when disabled
• test_precompact_enabled_by_default — precompact blocks even when stop is disabled
• test_stop_disabled_precompact_still_works — independence verified
• All 59 existing hook/config tests pass (0 regressions)

$ python -m pytest tests/test_config.py tests/test_hooks_cli.py -v
============================== 59 passed in 0.10s ==============================

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