#10 · stacks/uplink

Part of #4. The per-installation half shipped in #7; this is the per-thread half.

A global setting alone cannot answer "use Codex for this one thread" from a phone, which is the point of the product. A per-text prefix is easy to forget and has to survive both the filter and detectWorkdir. The control-word vocabulary already solves this shape of problem.

What is there today

UPLINK_ENGINE and a settings.json field choose the engine for the whole installation, and the menubar offers a picker (app/Uplink/engines.ts, app/Uplink/desktop-agent.ts). Nothing reaches that from a phone, and every thread on a Mac uses the same engine.

parseControl (app/Uplink/commands.ts#L24) matches only a whole message, after stripping a leading slash and trailing punctuation. That is why stop cancels a run while "stop the dev server in ~/Code/x" still reaches the agent as a task. ALIASES (app/Uplink/commands.ts#L8) is the whole vocabulary and HELP_TEXT (#L29) is what help texts back.

A conversation stores one sessionId and one cwd (app/Uplink/store.ts), and begin() resumes when the session id, the cwd and the idle window all agree (app/Uplink/uplink.ts#L299).

Proposal

Add claude and codex to ControlCommand (app/Uplink/commands.ts#L6) and ALIASES. A bare codex switches the thread and replies with confirmation; "codex keeps timing out on ~/Code/x" still reaches the agent, by the same rule that already protects stop.
Add engine to conversations and to runs as defineModel attributes, schema.enum(['claude', 'codex']), default 'claude' so existing rows read correctly. Run buddy generate:migrations. Mirror both in the downloadable app's SCHEMA and RUN_COLUMNS (app/Uplink/sqlite-store.ts), which needs the migration path from the separate issue first.
Switching engines must drop the stored session id. A Claude session id cannot be resumed by Codex and the reverse is equally true. The resume guard at app/Uplink/uplink.ts#L299 gains the engine to its list of things that must agree, and the control word clears sessionId the way new does at #L415.
Pin the engine on the Job (app/Uplink/uplink.ts#L46) at enqueue time, so a queued task runs on the engine it was sent to rather than whatever the thread switched to while it waited.
Add the two words to HELP_TEXT and to the homepage's list of words it always answers (resources/views/index.stx#L39).

Requirements

status says which engine is working. A thread that can switch needs to be able to say what it is on (app/Uplink/uplink.ts#L390).
Default stays Claude. A user who never texts codex sees no change at all.
An unavailable engine must say so when it is selected, not when the next text fails. If codex is texted and findCodex() found nothing, reply with how to install it.

Acceptance

A test texts codex, then a task, and asserts the run recorded engine = 'codex' and that no --resume or resume subcommand carried the Claude session id across the switch.
A test asserts a queued job keeps the engine it was enqueued with.
help lists both words.