[docs] Update Coder docs for runner-node picker, engines, and package paths

[serge-ivo]

Problem

Coder docs lag behind the current multi-machine and engine implementation.

The public multi-machine doc says UI work remains to choose connected nodes, but the Settings tab now exposes a Runs on picker backed by /v1/instances/:id/runner-node. The Coder README also points to an obsolete console file path.

Evidence

• Stale public doc: platform/platform-docs/coder-multi-machine.md
• Generated stale HTML: platform/store/docs/coder-multi-machine/index.html
• Runner-node API: platform/workers/api/src/routes/instances.ts
• Coding session honors requested/pinned runner node: platform/workers/api/src/routes/coding.ts
• Engine presets/auth live in platform/workers/api/src/lib/coding-engines.ts
• Actual UI package: platform/agents/coder/web/src/CodingTab.tsx
• Stale README path: platform/agents/coder/README.md says store/console/console-coding.js

Scope

• Update Coder multi-machine docs to describe the existing Runs on picker and /runner-node behavior.
• Document behavior when a pinned node is offline or online but not attached to the instance.
• Update Coder README architecture paths to @proagentstore/coder-web / agents/coder/web/src/*.
• Update engine docs for current defaults: Claude, Codex, Gemini, Grok, and local/Ollama where relevant.
• Regenerate public docs if platform-docs changes.

Acceptance Criteria

• Public Coder docs match the current UI/API behavior.
• No docs say runner-node picker UI is still future work.
• Coder README file paths match the repo.

[serge-ivo]

Status update after docs/code audit: partially fixed, keep open. platform-docs/coder-multi-machine.md now describes runner-node behavior and docs/coding-engines.md covers the current engines. Remaining concrete stale item: agents/coder/README.md still says the console lives at store/console/console-coding.js; it should point to @proagentstore/coder-web / agents/coder/web/src and the current console surface.

[serge-ivo]

Done in 0b7f6ea (direct to main).

platform-docs/coder-multi-machine.md — rewritten. It closed on "the remaining UI work is to show connected nodes and let users choose the node"; the Runs on picker has been shipped in the Settings tab, backed by GET/PUT /v1/instances/:id/runner-node. The doc now covers the picker and its three tile states, the connected vs nodeOnline distinction (machine off ≠ machine on but this agent not attached), pinned-is-authoritative routing (pinned + dead relay = offline, never a silent fallback to another machine), unpinned routing following the live relay socket rather than the stale status column, session reclaim on a machine switch, and the three diagnoseAttachment states with the command each one actually needs — including the case where pags up is the wrong advice and pags up --force is right.

agents/coder/README.md — architecture table repointed at @proagentstore/coder-web / agents/coder/web/src/*, plus lib/coding-engines.ts, lib/runner-client.ts, lib/runtime-nodes.ts, lib/runtime-attachment.ts, lib/coding-timeline.ts, and store/console/src/lib/surfaces.tsx. Also corrected what it said, not just where it pointed: the engine is a child process driven over stream-json and resumed with --resume from ~/.claude, not a tmux pane (tmux belongs to the terminal-operator agents); the coding_sessions.tmux_session column keeps its legacy name but stores the <engine>:<id> engineLabel. Added the two declarative Coders (coder-repo, coder-lead), the five shipped engine presets, sign-in modes + per-session credential reporting, start_work/check_work, per-instance loop presets, repo-scoped history, and the fact that pags up attaches new agents without a restart (CLI ≥ 0.4.30).

docs/coding-engines.md — added the shipped-defaults table. It listed four engines in its write-permission table and never named DEFAULT_ENGINES, so the local / Ollama preset appeared nowhere; also documents deriveClientType's mapping and why an unknown binary falls to codex (raw) rather than Claude.

pnpm docs:drift green before and after; pnpm typecheck and npx vitest run (229 files / 3253 tests) green.

Not covered here, noted for follow-up (all code/data, not docs):

• store/console/src/tabs/SettingsTab.tsx tells the user to restart pags up when a machine is online but the agent is not attached — --watch-instances (pags up should attach newly configured runner agents without restart #229) attaches within ~20s, so the advice predates it.
• Migration 0063's coder-repo engine setting still describes itself as "Which CLI this agent drives in tmux".