v0.3.0

Improvements to the Phase-5 sandbox feature, validated end-to-end against a real Dockerized Laravel/Sail/PostgreSQL app (full L1 to L4 ladder, including live per-sandbox PostgreSQL).

Features

• Configurable worktrees namespace (sandbox.project_name). Optional override for the ~/.atelier/worktrees/<name>/ namespace; when unset, derived from the repo's git common dir so a bare-repo/worktree layout no longer collides on a working tree named master/main. New git_helper.git_common_dir().
• Explicit branch override on the sandbox create form. An optional branch input (with inline validation) that wins over the auto-namer, for branch conventions the namer can't produce (e.g. uppercase ticket keys).

Fixes

• Roster prompt paths anchored at trace_root() for sandbox runs. A sandbox run's cwd is the worktree (which need not contain adws/); prompt files now resolve against the trace root like the db/data_dir sink, so runs no longer fail config validation before tracing. Non-sandbox runs unchanged.

Gates green (pnpm typecheck + pnpm check:contract; no db/seam change). cockpit/package.json bumped to 0.3.0.

Full details: #26