feat: Add worktree setup hooks for injecting local configurations (#458)

* feat: Add worktree setup hooks for injecting local configurations

Implements GitHub issue #220 - Worktree setup hook for injecting
local configurations.

When polecats are spawned, their worktrees are created from the rig's
repo. Previously, there was no way to inject custom configurations
during this process.

Now users can place executable hooks in <rig>/.runtime/setup-hooks/
to run custom scripts during worktree creation:

  rig/
    .runtime/
      setup-hooks/
        01-git-config.sh    <- Inject git config
        02-copy-secrets.sh  <- Copy secrets
        99-finalize.sh      <- Final setup

Features:
- Hooks execute in alphabetical order
- Non-executable files are skipped with a warning
- Hooks run with worktree as working directory
- Environment variables: GT_WORKTREE_PATH, GT_RIG_PATH
- Hook failures are non-fatal (warn but continue)

Example hook to inject git config:
  #!/bin/sh
  git config --local user.signingkey ~/.ssh/key.asc
  git config --local commit.gpgsign true

Related to: hq-fq2zg, GitHub issue #220

* fix(lint): remove unused error return from buildCVSummary

buildCVSummary always returned nil for its error value, causing
golangci-lint to fail with "result 1 (error) is always nil".

The function handles errors internally by returning partial data,
so the error return was misleading. Removed it and updated caller.

internal/polecat/manager.go

@@ -322,6 +322,13 @@ func (m *Manager) AddWithOptions(name string, opts AddOptions) (*Polecat, error)
 		fmt.Printf("Warning: could not copy overlay files: %v\n", err)
 	}
 
+	// Run setup hooks from .runtime/setup-hooks/.
+	// These hooks can inject local git config, copy secrets, or perform other setup tasks.
+	if err := rig.RunSetupHooks(m.rig.Path, clonePath); err != nil {
+		// Non-fatal - log warning but continue
+		fmt.Printf("Warning: could not run setup hooks: %v\n", err)
+	}
+
 	// NOTE: Slash commands (.claude/commands/) are provisioned at town level by gt install.
 	// All agents inherit them via Claude's directory traversal - no per-workspace copies needed.