Default flip: Pi first with probe-order fallback + gate recertification policy (Phase 3 of #270)

[ElbertePlinio]

Phase 3 of #270 (Make Pi the default agent). Flip the default so Pi is the first option, with Claude Code and Codex as secondaries — honoring the umbrella's locked decisions: default ≠ only, existing explicit selections never migrated, graceful fallback when pi is absent or gate-incompatible.

Depends on: #397 (do not flip while PI_CAPABILITIES still stubs thinking-level, image input, or context reporting — "a default that demotes UX ships never").

Current state (verified 2026-07-27)

• src/lib/chatDefaults.ts falls back to "claudeCode" in both selectableLastAgentProvider and loadLastAgentProvider.
• selectableNativeAgentBackends (src/lib/agentBackends.ts) appends Pi after Claude/Codex, and only when isCompatiblePiRpcVersion passes.
• Quick-launch chips have no Pi-first default.
• The certified gate pins 0.79.10 ≤ v ≤ 0.81 (TS + Rust, kept in sync per the Make Pi the default agent for PickForge #270 Phase 0 boundary-matrix test). Installed today: 0.81.1 (passes). Once Pi is the default, any Pi release beyond the pin silently drops new installs back to Claude until recertified — the stale-pin trap Make Pi the default agent for PickForge #270 Phase 0 already hit once.

Scope

1. Probe-order default: first-run/new-project default resolves pi → claude → codex; incompatible or missing pi falls back with a one-line hint, never a blocking prompt. Existing pickforge.lastAgentProvider values untouched.
2. Ordering: Pi listed first in selectable backends and pickers when the gate passes.
3. Quick-launch chips: Pi chip first, pinned to a cheap default model consistent with the chips policy in CLAUDE.md.
4. Gate recertification policy: write down the workflow (headless RPC contract probe + micro dogfood, as run for 0.81) and where it lives, so widening the pin per Pi release is a routine small PR, not an investigation. Decide whether an installed-Pi-newer-than-pin state surfaces a hint in Settings.
5. Copy + docs: Settings copy, onboarding hints, repo docs.
6. Success metric (telemetry-free): define what convinces us the default sticks before closing Make Pi the default agent for PickForge #270 Phase 3.

Agent plan

Checklist:

• Probe-order default + fallback hint + tests (no migration of stored selections — regression test)
• Backend ordering + picker order + tests
• Quick-launch chip default + tests
• Gate recert workflow documented; Settings hint decision recorded
• Copy/docs pass
• Success metric written into Make Pi the default agent for PickForge #270
• PR/review

Validation:

• bun run build, bun run test:unit, bun run vrt (picker/chips order), cargo test --workspace --locked --all-targets
• Manual: fresh-profile first run with pi present, with pi renamed away (fallback + hint), and with a stored codex selection (untouched)

PR plan

• PR 1 — defaults + ordering + chips flip with fallback (single behavioral outcome)
• PR 2 — gate recert doc + Settings hint (if adopted) + copy/docs

Refs #270, #212

Current status: Planned (blocked on Phase 2)
Next action: wait for Phase 2, then PR 1