feat(rig): add manifest support for rig configuration

[easel]

This PR introduces the ability for a rig to describe what resources it wants from a town. The intent is to get it down to a oneliner to, for instance, add rigs for gastown and beads and start a crew so you can build from main and contribute fixes.

Summary

• Add .gt/rig.toml manifest file support for standardizing rig setup
• Implement fork detection and upstream/origin remote configuration
• Add crew presets with agent, model, and account support
• Add gt rig update command for checking/pulling updates
• Support setup command execution from manifest

Details

The manifest enables one-liner rig installation with automatic crew workspace creation and proper remote configuration. Key features:

• Manifest parsing: .gt/rig.toml defines rig defaults (name, prefix, branch), git settings (upstream, fork policy), setup commands, and crew configurations
• Fork workflow: Detects existing forks, prompts to create/use fork, configures remotes correctly
• Crew presets: Supports agent type, model, account, branch, args, and env configuration per crew member
• Update command: gt rig update --check reports ahead/behind status, --pull updates and runs setup

Test plan

• go build ./... passes
• go test ./... passes
• gofmt clean
• go vet clean
• Manual testing of gt rig add with manifest
• Manual testing of fork detection flow
• Manual testing of gt rig update

🤖 Generated with Claude Code

[easel]

Resubmitted as #727 (head branch was main; new branch pr/rig-manifest rebased onto upstream/main).