feat(config): add watcher_enabled config key to disable the background watcher (#335)

[nguiaSoren]

Summary

Adds a persistent watcher_enabled config key (default true) that fully
disables the background watcher when set to false. Closes #335.

Per the maintainer's contract on #335: watcher_enabled defaults to true,
false prevents the watcher thread from starting and from registering
projects, and manual index_repository continues to work normally. Scope is
limited to this one switch — no other watcher policy changes.

Why (and how it differs from auto_watch)

auto_watch (default true) already gates per-session registration — but
the watcher thread still runs idle even with auto_watch=false. #335 asks
for a way to stop the watcher entirely (the doc-heavy-repo use case: every save
churns the graph baseline). watcher_enabled is that master switch: when
false, main() never creates the watcher, so the poll thread never starts and
nothing is registered. auto_watch is left unchanged as the finer-grained
per-session registration control. Both are now documented together.

(Note: the watcher polls for git changes now, not raw mtime as the original
0.6.0-era issue described; the user-visible concern — auto-reindex churn — is the
same, and this switch addresses it.)

The change

• src/cli/cli.h / src/cli/cli.c — CBM_CONFIG_WATCHER_ENABLED
  ("watcher_enabled") plus a small NULL-safe predicate
  cbm_config_watcher_enabled(cfg) (returns the persisted bool, default true).
  Listed in config usage help and config list output, alongside the sibling
  keys.
• src/main.c — gate watcher creation + set_watcher on the predicate; the
  existing thread-create is already gated on g_watcher, so a disabled watcher
  means no thread and (via the existing NULL guard in
  register_watcher_if_enabled) no registration. cbm_mcp_server_set_config
  stays outside the gate so auto_index / auto_watch still read config when
  the watcher is off. Logs watcher.disabled reason=config when skipped.
• tests/test_cli.c — cli_config_watcher_enabled_default_and_persist
  drives the predicate main() gates on: default on, false/0/off disable,
  true/1/on re-enable, persistence across reopen, and a NULL-config default
  (a config-store failure must never silently disable the watcher). The existing
  dump_verify-style tests feed synthetic values; this exercises the real seam.
• docs/CONFIGURATION.md / README.md — document watcher_enabled, and add
  the previously-missing auto_watch row so the watcher knobs are described
  together.

Verification

• Clean scripts/test.sh (ASan + UBSan) green: 6329 passed, 1 skipped,
  including the new test.
• Behavioral proof on the built binary (isolated CBM_CACHE_DIR):
  config set watcher_enabled false → server logs watcher.disabled reason=config and no watcher thread starts; default/true → watcher starts
  normally (watcher.disabled count 0); manual index_repository unaffected.
• No Makefile.cbm change (all touched files already compiled); no config set
  allowlist to update; no CHANGELOG in the repo.

All commits are DCO signed-off.