feat(watcher): opt-in polling of non-git project roots (#1948)

[DeusData]

A project indexed from a directory that is not a git repository was never
refreshed after its first index: init_baseline classified it is_git=false
and poll_project/check_changes returned early for it forever, so the graph
silently went stale. The comment in init_baseline even claimed such folders
were "polled as a plain directory", which was never true.

Add an opt-in config key, watch_non_git (default false, read once at daemon
startup like watcher_enabled). When on, a non-git root is polled on the
existing adaptive cadence (cbm_watcher_poll_interval_ms) by a tree
signature: the indexer's own cbm_discover walk (same skip lists, .gitignore
stack and .cbmignore), sorted by path and folded over (relative path, size,
mtime). A changed signature takes the existing reindex path with the same
commit-after-success discipline as the #937 dirty signature. The baseline
leaves the signature "unknown", so the first poll reindexes once
(at-least-once, covering edits made while the daemon was down).

Runaway guard: only files discovery would index enter the signature, so
cbm's own .codebase-memory output (built-in skip) and the cache directory
(pruned by absolute path) can never make a finished reindex look like a
new change; a scratch folder under an unrelated dirty repository is polled
on its own files and never inherits the ancestor's dirty state.

cbm_file_info_t gains mtime_ns, filled from the stat discovery already
performs, so the signature needs no second stat per file and stays
UTF-8-correct on Windows (discovery's wide stat).

Docs: README and docs/CONFIGURATION.md now say non-git roots are not
watched by default and document watch_non_git. The staleness signal in
index_status is left to #1561's freshness object. Refs #1948.