refactor(server): share worktree create/list commands

[boudra]

Summary

• Add shared worktree create/list commands to packages/server/src/server/worktree/commands.ts.
• Route Session worktree list/create handlers through the shared command surface while keeping WS response/update shaping in worktree-session.ts.
• Route MCP list_worktrees / create_worktree through the same command surface while keeping MCP target parsing and structuredContent shaping in mcp-server.ts.

Risk / coverage notes

• Session list branch: covered by packages/server/src/server/worktree-session.test.ts (handlePaseoWorktreeListRequest > lists worktrees through the workspace git service). This caught and fixed an exact call-shape drift during the refactor.
• Session create success, workspace update, setup continuation, git warmup, and error-code branches: covered by packages/server/src/server/worktree-session.test.ts create/workflow tests.
• MCP create target validation and create/list tool behavior: covered by packages/server/src/server/agent/mcp-server.test.ts.
• Lower-level registered worktree side effects remain covered by packages/server/src/server/paseo-worktree-service.test.ts; this PR does not move that service behavior.
• MCP real worktree create/list remains indirectly covered by packages/server/src/server/agent/mcp-parity.e2e.test.ts, not rerun locally per targeted-test rule.

Verification

• npx vitest run packages/server/src/server/worktree-session.test.ts --bail=1 (pre-refactor: green; post-refactor: one call-shape drift found, fixed; rerun green)
• npx vitest run packages/server/src/server/agent/mcp-server.test.ts --bail=1 (pre-refactor green; post-refactor green)
• npm run format
• npm run typecheck
• npm run lint

Unslop check

• Kept transport-specific response/error payload shaping in Session and MCP.
• Removed the MCP-only create wrapper instead of adding another coordinator.
• Command deletion test: deleting the new create/list command paths makes direct listWorktrees, runSetup: false, paseoHome forwarding, and worktree error conversion reappear in both Session and MCP.

[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.