feat(hooks): add per-hook auto-save controls

[MichaelWave369]

Closes #2294.

Keeps the existing hooks.auto_save / MEMPALACE_HOOKS_AUTO_SAVE behavior as the master switch and adds independent per-hook opt-outs in config.json:

{
  "hooks": {
    "stop": false,
    "pre_compact": true,
    "session_end": true
  }
}

Behavior:

• omitted per-hook keys default to enabled, so existing configs are unchanged;
• master auto_save: false still disables all auto-save hooks;
• an environment master override still composes with the per-hook settings;
• non-boolean per-hook values do not silently disable capture;
• disabling SessionEnd still preserves its existing per-session save-marker cleanup.

The new check lives at the existing hook dispatcher, so a disabled hook returns before its Stop / PreCompact / SessionEnd handler performs checkpoint or mining work. The existing handler-level master-switch behavior remains unchanged.

Regression coverage in tests/test_per_hook_auto_save.py pins:

• default enablement;
• independent per-hook controls;
• master-switch precedence;
• environment-master precedence;
• malformed per-hook values;
• Stop and PreCompact dispatch short-circuiting;
• SessionEnd cleanup while disabled;
• normal dispatch when enabled.

Static branch verification against current develop (639c69a):

• one commit ahead, zero behind;
• two changed files;
• runtime change in mempalace/hooks_cli.py: +45 / -0;
• standalone focused regression test file added.

I was not able to execute the project test suite in the current contributor environment, so I am leaving execution to upstream CI rather than claiming a local pass I did not perform.