Integrate the pi.dev coding harness as a coder subagent

[mattmezza]

Summary

Add the ability to invoke a coding harness (pi.dev — an open-source, provider-agnostic terminal
coding agent) for software tasks, wired as a specialised subagent rather than a new subsystem.

Why it fits

pi exposes non-interactive modes intended for embedding: one-shot print (-p / --print) and
newline-delimited JSON events (--mode json), plus RPC/SDK. It is provider-agnostic and works with
the same providers the agent already uses. This is the same product-layer-over-harness-layer shape
already seen elsewhere in the ecosystem.

Approach

• Add pi to the executor whitelist and an entry in the optional-tools registry (provider-key env
  injection, mirroring the gh token pattern).
• A coding.md skill documents invocation and JSON-event parsing.
• Wire it as the coder subagent specialisation: spawn_subagent(persona="coder", ...) shells into
  pi (print / JSON mode) instead of running the agent's own loop.
• Coding tasks are long-running → use the async subagent path and a per-task workspace dir under
  data/.

Security (hard requirement)

pi has no built-in permission system and runs with the permissions of the launching process; its own
docs recommend containerisation. Therefore pi MUST run inside the sandbox sidecar, never in the main
process. The agent's permission engine gates invoking pi (ASK before a run); the container gates
what pi can touch.

UX & product

• Admin UI: enable/disable toggle, a provider-key field (stored in the vault), and a coding-run
  viewer showing streamed JSON events / full logs and the produced workspace/diff — responsive/
  touch-friendly at phone width, reusing consistent toggle + masked-field + log-viewer components.
• On the go (Telegram): dispatch a coding task in chat; receive a concise summary (what
  changed, pass/fail) rather than a wall of logs; an inline approval gates the run. Consistent
  progress/approval conventions.
• Mobile-first: summaries-not-logs keeps coding usable on a phone; full output stays in the web
  UI.

Setup & onboarding

• Disabled by default; an optional wizard step provisions the sandbox sidecar and the provider
  key via the vault when enabled.

Acceptance criteria

• A coding task can be dispatched to pi non-interactively and its result returned/streamed.
• A coding task's outcome can be understood from a Telegram summary on mobile.
• pi executes only inside the sandboxed sidecar with a scoped workspace.
• Invoking a coding run requires approval.

Related

• Depends on: subagents, and the sandbox sidecar from the browser-automation issue.
• Provider key stored via: secrets vault.