Fixes #52453
Fixes #53246

Both issues were introduced by #51208 ("Handle Linux FS Rescan Events"),
which added `PathEventKind::Rescan` handling.

## Rules files not loading (#52453)

`load_worktree_info_for_system_prompt` called `load_worktree_rules_file`
synchronously, which uses `entry_for_path()` on the current worktree
snapshot. If the background scanner hasn't finished its initial scan,
the entry doesn't exist yet and the lookup returns `None` — the code
concludes no rules file exists. This was always a latent race condition,
but became more visible after Rescan events were introduced, since they
can trigger additional `WorktreeUpdatedEntries` churn that interacts
with the refresh mechanism.

The fix awaits `scan_complete()` on local worktrees before performing
the rules file lookup, ensuring the full directory tree is indexed
first.

## Config file rescan clearing OAuth tokens (#53246)

The `Rescan` handlers in `watch_config_dir` used
`fs.load(file_path).await.unwrap_or_default()`, which turns any
file-read error into an empty string. This empty string flows to
consumers like `CopilotChat`, where `extract_oauth_token("")` returns
`None`, causing the OAuth token to be unconditionally overwritten with
`None` — triggering re-authentication.

The fix changes both Rescan handlers to skip files that fail to load
(using `if let Ok(contents) = ...`), matching the pattern already used
by the `Created`/`Changed` handler.

Release Notes:

- Fixed rules files (AGENTS.md, CLAUDE.md, .rules, etc.) sometimes not
being applied in agent threads.
- Fixed GitHub Copilot re-prompting for authentication after filesystem
rescan events.