feat: slash commands and goal halt-gate

[arjunkmrm]

Summary

• New JSX emit kind: emitCommand({name, description?, handler}). Collected by the render walker into Rendered.commands alongside fragments and tools.

• Slash-command router in agent.run: a string input matching /<name>(\s+args)? is routed to the matching command's handler instead of inference. Unknown slash inputs fall through to inference with a warning.

  // tree: <Agent><Goal /><Workspace root="./" /><Messages /></Agent>
  
  await agent.run("/goal tests are green")
  //   → matches the "goal" command; handler registers a halt predicate
  //   → no user.message appended, no inference triggered
  //   → returns once handler resolves
  
  await agent.run("/goal clear")
  //   → same command, args = "clear"; handler unregisters the predicate
  
  await agent.run("fix the failing test")
  //   → no slash prefix → normal path: user.message appended, inference fires
  
  await agent.run("/typo something")
  //   → no "typo" command registered → warns
  //     "[agentjsx] received slash input \"/typo\" but no command with
  //      that name is registered; passing through as user.message"
  //   → falls through to inference with the literal "/typo something"

• Halt-predicate registry on AgentCtx: registerHaltPredicate, clearHaltPredicate, getHaltPredicates. Stored as a SubscriptionRef<Map<string, HaltPredicate>> so future consumers can subscribe.

• runHaltGate(infer) supervisor: forked scoped fiber that watches the event log for "about to return control to the user" states — both explicit assistant.halted and natural terminals (last event = assistant.message with no pending tool calls). Runs all registered predicates; on any ok: false, appends a synthetic user.message with the failing reasons so the inference loop reprompts the model.

• <Goal /> component: ships the /goal <condition> (and /goal clear) slash command. The handler registers a HaltPredicate that judges the transcript against the condition via a separate infer call, returning {ok, reason} JSON.

• <Skills> reflects commands: frontmatter into emitCommand entries — each skill's commands become slash invocations whose handlers append the skill's prompt as a user message.

Test plan

• pnpm check-types — clean.
• pnpm test — 45 files, 166 tests pass.
• test/agentctx/core/agent-run-contract.test.ts (5 tests): non-slash input goes to inference; unknown slash falls through with warning; registered command's handler runs and the slash input is not appended as user.message; assistant.halted is terminal without predicates; halt-gate reprompts when a predicate returns ok=false.
• test/agentctx/core/goal-component.test.ts: <Goal> emits one fragment and one command with name "goal".
• Live e2e against sonnet-4-6 with a temporary <Goal />-based example: set /goal "greeted in pirate speak", agent says "Hello!" → halt-gate runs judge → ok=false, reason "no pirate speak" → reprompt appended → agent re-infers "Ahoy, matey!" → judge ok=true → halt stands.