feat(vscode): add native SDD agents

[Snakeblack]

🔗 Linked Issue

Closes #504

Related context: #677

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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

PR 1 of the VS Code Copilot SDD subagents stack. This adds the backend/native-agent foundation only: VS Code now supports native SDD .agent.md files in ~/.copilot/agents, with embedded coordinator/phase assets, injection post-check support, idempotency coverage, and uninstall preservation for user-authored agents.

Follow-up PRs will add VS Code model assignment foundation and TUI/docs. Those are intentionally out of scope here.

This PR requests size:exception: it exceeds 400 changed lines because it ships substantive Markdown .agent.md assets, not because the code surface is large.

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📂 Changes

File / Area	What Changed
internal/agents/vscode	Enabled native sub-agent support at ~/.copilot/agents and added path tests
internal/assets	Embedded all:vscode and added VS Code SDD .agent.md assets
internal/components/sdd	Accepted .agent.md in native sub-agent post-check and covered VS Code injection/idempotency
internal/components/uninstall	Verified uninstall removes only managed VS Code SDD agents and preserves custom .agent.md files

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🧪 Test Plan

Focused PR1 tests

go test ./internal/agents/vscode -run TestSubAgentSupportUsesCopilotUserAgentsDir
go test ./internal/assets -run TestVSCodeNativeAgentAssetsFrontmatter
go test ./internal/components/sdd -run 'TestInjectVSCodeWritesNativeAgentFiles|TestInjectNativeSubAgentExtensionsRemainAdapterSpecific'
go test ./internal/components/uninstall -run TestComponentOperationsSDD_VSCodeRemovesOnlyManagedAgentFiles

Package smoke

go test ./internal/agents/vscode ./internal/assets

Unit Tests

go test ./...

E2E Tests (Docker required)

cd e2e && ./docker-test.sh

• Unit tests pass (go test ./...)
• E2E tests pass (cd e2e && ./docker-test.sh)
• Manually tested locally

Notes: focused PR1 tests passed locally. Full go test ./... was not claimed here due known local Windows/CRLF/symlink/environment failures unrelated to this PR.

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🤖 Automated Checks

The following checks run automatically on this PR:

Check	Status	Description
Check PR Cognitive Load	⚠️	Requests size:exception; most added lines are .agent.md assets
Check Issue Reference	⏳	PR body contains Closes #504
Check Issue Has status:approved	⏳	#504 has status:approved
Check PR Has type:* Label	⏳	type:feature requested (maintainer label needed)
Unit Tests	⏳	CI should run go test ./...
E2E Tests	⏳	CI should run Docker E2E if configured

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✅ Contributor Checklist

• PR is linked to an issue with status:approved
• PR stays within 400 changed lines, or I have requested/obtained maintainer-applied size:exception with rationale documented — maintainer label needed (size:exception)
• I have added the appropriate type:* label to this PR — maintainer label needed (type:feature)
• Unit tests pass (go test ./...)
• E2E tests pass (cd e2e && ./docker-test.sh)
• 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

This is the backend/native-agent slice only. It deliberately does not include VS Code model assignment state, dynamic Copilot model catalog integration, TUI/profile UX, or final docs updates; those will come in follow-up PRs.

[Snakeblack]

Maintainer label request: please apply exactly one type label, type:feature, and size:exception for the Markdown-heavy VS Code .agent.md asset payload. Follow-up PRs will add model assignment foundation and TUI/docs.