feat(pi): add first-class PI support

[felipe3dfx]

🔗 Linked Issue

Closes #363

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🏷️ PR Type

What kind of change does this PR introduce?

• type:bug — Bug fix (non-breaking change that fixes an issue)
• type:feature — New feature (non-breaking change that adds functionality)
• type:docs — Documentation only
• type:refactor — Code refactoring (no functional changes)
• type:chore — Build, CI, or tooling changes
• type:breaking-change — Breaking change (fix or feature that changes existing behavior)

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📝 Summary

• Adds first-class PI agent support, including detection, install/sync planning, generated PI SDD assets, and compatibility documentation.
• Adds PI model picker support with separate PI/OpenCode assignment persistence, preselection from state, and source-specific sync/injection plumbing.
• Hardens PI RPC model loading for noisy extension startup output and adds regression coverage for mixed PI/OpenCode flows.

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📂 Changes

File / Area	What Changed
internal/agents/pi/*	Adds PI adapter, detection, paths, compatibility checks, Engram extension validation, and RPC model catalog loading.
internal/assets/pi/agents/*	Adds PI-native SDD subagent and chain assets.
internal/components/sdd/*	Adds PI SDD injection paths, PI-specific model assignment handling, and golden/regression coverage.
internal/tui/*	Adds PI model configuration flow, dual OpenCode+PI picker sequencing, PI assignment preselection, and UI warnings.
internal/state/*, internal/app/*, internal/cli/*	Persists and hydrates PI model assignments separately from OpenCode; wires install/sync flows.
internal/opencode/*, internal/modelcatalog/*	Adds normalized model catalog support and wrapped OpenCode provider parsing.
docs/pi-compatibility.md, docs/*, README.md	Documents PI compatibility and platform behavior.
testdata/pi/*	Adds PI sample config fixtures.

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🧪 Test Plan

Unit Tests

timeout 8m go test -count=1 -timeout=6m ./...

E2E Tests (Docker required)

cd e2e && ./docker-test.sh

• Unit tests pass (go test ./...) — pending final local/CI validation before marking ready for review
• E2E tests pass (cd e2e && ./docker-test.sh) — pending
• Manually tested locally

Additional focused validations already run during development:

go test ./internal/tui ./internal/app ./internal/state ./internal/cli
go test ./internal/agents/pi ./internal/tui
go test ./internal/app ./internal/cli ./internal/state ./internal/model
go test ./internal/tui ./internal/components/sdd ./internal/state

Manual checks performed:

• PI install flow reaches PI model picker.
• PI RPC model loading works via stdin request to pi --mode rpc despite extension UI events.
• PI model assignments persist to ~/.gentle-ai/state.json under pi_model_assignments.
• Re-opening PI model configuration preselects assignments from state.

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🤖 Automated Checks

The following checks run automatically on this PR:

Check	Status	Description
Check Issue Reference	⏳	PR body must contain Closes/Fixes/Resolves #N
Check Issue Has status:approved	⏳	Linked issue must have been approved before work began
Check PR Has type:* Label	⏳	Exactly one type:* label must be applied
Unit Tests	⏳	go test ./... must pass
E2E Tests	⏳	cd e2e && ./docker-test.sh must pass

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✅ Contributor Checklist

• PR is linked to an issue with status:approved — draft pending issue approval (#363 currently needs approval)
• I have added the appropriate type:* label to this PR
• Unit tests pass (go test ./...) — pending final validation
• E2E tests pass (cd e2e && ./docker-test.sh) — pending
• I have updated documentation if necessary
• My commits follow Conventional Commits format
• My commits do not include Co-Authored-By trailers

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💬 Notes for Reviewers

Draft PR while we complete final validation and wait for #363 to receive status:approved.

Areas worth reviewing closely:

• PI/OpenCode model assignment separation (model_assignments vs pi_model_assignments).
• PI RPC model loading behavior with extension UI events before catalog response.
• Sync/install state hydration and PI SDD artifact generation.