feat: add wt step eval for template expression evaluation

[max-sixty]

Summary

• Add experimental wt step eval subcommand that evaluates a minijinja template in the current worktree context and prints the result to stdout
• Reuses existing hook template infrastructure — all variables (branch, repo, worktree_path, etc.) and filters (hash_port, sanitize, sanitize_db) are available
• Output goes to stdout with no decoration or shell escaping, designed for scripting and shell substitution

$ wt step eval '{{ branch | hash_port }}'
16066

$ curl http://localhost:$(wt step eval '{{ branch | hash_port }}')/health

This is the first step from the IDEAS.md design doc, directly addressing the user request in #947 — "found no way to run it on a single branch."

Test plan

• 6 integration tests: basic branch, hash_port, multiple values, sanitize_db, conditionals, template error
• Help text and completion snapshots updated
• stdout guard allowlisted
• Doc pages auto-synced
• All lints pass
• 2360 tests pass (5 pre-existing shell_wrapper failures excluded)

Ref #947

This was written by Claude Code on behalf of @max-sixty

[max-sixty]

Follow-up PR for per-branch kv state: #1006

This was written by Claude Code on behalf of @max-sixty