Fix rules files not loading and config file rescan clearing tokens

[AJenbo]

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.