Adapter: send a message into a thread, archive it and remove its worktree #6

Outcome

Factory code can send a follow-up into a session, and close a finished session without leaving its worktree on disk.

Acceptance criteria

• FactorySessions.send(threadId, commandId, text) dispatches message.dispatch with the mode softfac's send uses (src/t3.rs). The same commandId twice sends one message.
• FactorySessions.archive(threadId) archives the thread. An unknown thread counts as already gone, and archiving an archived thread changes nothing; neither is an error.
• FactorySessions.removeWorktree(path) removes a thread's worktree when it is still on disk (git worktree remove --force, one retry on Directory not empty) and keeps the branch, as softfac's workspace.rs does.
• Contract tests with the test provider: a send after the first run completes starts a new run whose last message status reads; after archive, thread reports archived; after removeWorktree, the directory is gone and the branch still exists.
• factory/docs/design.md open question 4 is answered: what T3 does with a thread's worktree when the thread is archived.

Depends on

• Adapter: find a project, launch a thread and read its status in-process #5

Notes

Reference

• softfac, the Rust factory this ports, pinned at rawphp/softfac@bf273a1. Read it from a separate clone: git clone --filter=blob:none https://github.com/rawphp/softfac /tmp/softfac-ref && git -C /tmp/softfac-ref checkout bf273a1d93651992b4fc95b4dd6c111f8f3322cf. Its docs/lifecycle.md, docs/reference/ledger.md and docs/issues.md describe the behaviour; its tests pin it.
• Design: factory/docs/design.md in this repo.
• Upstream's AGENTS.md applies. Run only the tests and typechecks for the paths you touch (vp test run <path>, vp run --filter t3 typecheck), never the repo-wide checks.
• Edit no upstream file that is not listed in factory/SEAMS. A new seam needs a line in factory/SEAMS and a reason in the PR body.
• Port behaviour, not code: port the named test cases first (TypeScript, vitest / @effect/vitest), then make them pass. Exact comment and ledger wording is ported as is.