refactor(server): extract worktree archive command

[boudra]

Summary

• Extract shared archivePaseoWorktreeCommand for common worktree archive target resolution, ownership checks, and archive service invocation.
• Make Session and MCP archive handlers entrypoint adapters: Session emits WS response shapes; MCP returns structured content and keeps MCP argument errors local.
• Add MCP coverage for archiving a worktree by slug.

Risk and Coverage Notes

• Low risk: archive teardown/delete side effects stay in archivePaseoWorktree, covered by worktree-session.test.ts archive cases for agent close, terminal teardown, archiving state updates, failure cleanup, orphaned git metadata, and snapshot refresh.
• Low risk: MCP archive side effects remain covered by mcp-server.test.ts for snapshot refresh and workspace archiving callbacks.
• Newly covered: MCP archive_worktree with worktreeSlug, proving the shared command preserves slug-to-Paseo-worktree-path behavior and deletes the worktree.
• Intentional transport difference: Session maps NOT_ALLOWED to paseo_worktree_archive_response; MCP throws the tool error. Both cross the same command result.

Verification

• npx vitest run packages/server/src/server/worktree-session.test.ts packages/server/src/server/agent/mcp-server.test.ts --bail=1
• npm run format
• npm run typecheck
• npm run lint
• pre-commit hook: lint, format:check:files, typecheck

Refactor Candidate

Chosen: shared archive worktree command.

The deleted duplication was target resolution plus Paseo ownership policy in Session and MCP. Deleting the new command would make worktree-session.ts regain branch lookup and ownership checks, and mcp-server.ts regain slug/path construction plus archive service assembly.

Next likely slice: standalone create/list worktree commands, if a deletion test shows enough remaining drift beyond the existing createPaseoWorktreeWorkflow.

[boudra]

Closing in favor of the single integration PR #893, which contains the full Session/MCP command-boundary stack and is being held until after the stable release.