perf(tmux): batch session queries in gt down (#477)

* perf(tmux): batch session queries in gt down to reduce N+1 subprocess calls

Add SessionSet type to tmux package for O(1) session existence checks.
Instead of calling HasSession() (which spawns a subprocess) for each
rig/session during shutdown, now calls ListSessions() once and uses
in-memory map lookups.

Changes:
- internal/tmux/tmux.go: Add SessionSet type with GetSessionSet() and Has()
- internal/cmd/down.go: Use SessionSet for dry-run checks and session stops
- internal/session/town.go: Add StopTownSessionWithCache() variant
- internal/tmux/tmux_test.go: Add test for SessionSet

With 5 rigs, this reduces subprocess calls from ~15 to 1 during shutdown
preview, saving 60-150ms of execution time.

Closes: gt-xh2bh

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* perf(tmux): optimize SessionSet to avoid intermediate slice allocation

- Build map directly from tmux output instead of calling ListSessions()
- Use strings.IndexByte for efficient newline parsing
- Pre-size map using newline count to avoid rehashing
- Simplify nil checks in Has() and Names()

* fix(sling): restore bd cook directory context for formula-on-bead mode

The bd cook command needs to run from the target rig's directory to
access the correct formula database. This was accidentally removed
in a previous commit, causing TestSlingFormulaOnBeadRoutesBDCommandsToTargetRig
to fail.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

internal/cmd/sling.go

@@ -380,8 +380,9 @@ func runSling(cmd *cobra.Command, args []string) error {
 		formulaWorkDir := beads.ResolveHookDir(townRoot, beadID, hookWorkDir)
 
 		// Step 1: Cook the formula (ensures proto exists)
-		// Cook doesn't need database context - runs from cwd like gt formula show
+		// Cook runs from rig directory to access the correct formula database
 		cookCmd := exec.Command("bd", "--no-daemon", "cook", formulaName)
+		cookCmd.Dir = formulaWorkDir
 		cookCmd.Stderr = os.Stderr
 		if err := cookCmd.Run(); err != nil {
 			return fmt.Errorf("cooking formula %s: %w", formulaName, err)