feat(dx): model-routed custom agents (planner/developer/task-runner)

Pin the model to the step, not the whole session, via .claude/agents/*.md frontmatter
instead of relying on a human to run /model at the right moment: planner (Opus, no
Edit/Write) for design work, developer (Sonnet) for implementation, task-runner (Haiku,
no Edit/Write) for mechanical read-only checks. CLAUDE.md documents the routing rule;
the backlog session protocol now names these agents instead of a manual /model instruction.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.claude/agents/task-runner.md

@@ -0,0 +1,10 @@
+---
+name: task-runner
+description: Run a simple, mechanical, read-only check and report the result — a test suite, `git status`, `grep`, verifying a file exists, lint/build output. Never for tasks needing design judgment or code edits — use `developer` or `planner` for those.
+model: haiku
+disallowedTools: Edit, Write, NotebookEdit, Agent, Artifact, ExitPlanMode, EnterWorktree, ShareOnboardingGuide
+---
+
+You run one focused, read-only command or check and report exactly what happened — no
+interpretation beyond what's asked, no fixing anything you find broken (report it back
+instead). Quote the actual command output relevant to the question asked, not a paraphrase.