v0.18.0 — worktree-isolated pipeline flow

Worktree-isolated pipeline flow + optional PR (feature 004)

Every bender pipeline session now runs inside a dedicated git worktree on a
dedicated session branch (bender/session/<id>). The main working tree is
never written to by bender; concurrent sessions do not collide. Worktree
isolation is mandatory — incompatible repos (git missing, bare,
mid-rebase / merge / cherry-pick) refuse to start with a specific exit code.

Pull requests are strictly opt-in via bender sessions pr <id>: no push,
no PR, no remote mutation without explicit invocation. Uses the user's
locally installed platform CLI (gh for GitHub today; glab stubbed for a
future release).

New CLI

• bender worktree create <session-id> [--base-branch=BRANCH]
• bender worktree list [--json]
• bender worktree remove <session-id> [--force]
• bender worktree prune [--older-than=DURATION]
• bender sessions pr <session-id> [--draft] [--refuse-update] [--json]
• bender sessions list gains WORKTREE / BRANCH / PR columns (v1 sessions labeled legacy)

Schema (backwards-compatible)

• state.json v1 → v2: adds worktree, session_branch, base_branch, base_sha, optional pull_request
• v1 files continue to load (flagged as legacy in the UI; no migration needed)
• 5 new event kinds: worktree_created, worktree_removed, worktree_missing, pr_opened, pr_update_refused

Exit-code table

Code	Meaning
10	git unavailable / repo bare / not a git repo
11	repo mid-rebase / mid-merge / mid-cherry-pick
12	configured worktree root violates placement rules
13	session branch bender/session/<id> already exists
20	remove/pr: session does not exist
21	remove: refused on an active session
30	pr: session not eligible (active / no commits)
31	pr: no adapter matches the configured remote
32	pr: push or platform-CLI call failed
33	pr: refused to update existing PR under --refuse-update

New packages

• internal/worktree — lifecycle primitives, GitRunner + ExecRunner + FakeRunner
• internal/pr — Adapter interface, GitHub adapter via gh, GitLab stub

Not affected

• Existing commands, skills, and pipeline contracts are untouched.
• v1 sessions on disk continue to load — no migration required.

See the README bender worktree and bender sessions pr sections for the
full walkthrough.