refactor: memoize autotrack probes, dedupe lock policy and logs

[ScriptedAlchemy]

Follow-up to #377 (merged): small reviewed cleanups in the PR-autotrack daemon path.

• Memoize environment probes: gh_available() is now cached process-wide in a OnceLock<bool> and origin_is_github() per repo root in a static Mutex<HashMap<PathBuf, bool>>, instead of re-spawning gh --version / git remote get-url origin every poll cycle to re-decide constants.
• Drop the unused SyncConfig parameter from poll_project.
• Extract shared BRANCH_LOCK_RETRY_ATTEMPTS / BRANCH_LOCK_RETRY_INTERVAL consts in src/branch.rs, used by both the async prepare_branch_tracking_in_layout retry loop and the sync acquire_branch_add_lock_blocking (loop bodies stay separate).
• Extract a log_pr_skip helper for the four duplicated "skipped" log_daemon_event blocks in reconcile_project (field set and ordering unchanged).

Verified: cargo fmt --check, cargo clippy --workspace --all-targets --locked -- -D warnings (0 errors), cargo nextest run --test daemon_suite --no-fail-fast (18 passed), cargo test --lib pr_autotrack (12 passed), cargo test --lib branch (48 passed).

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