Localize ID prefixes

[kitlangton]

Summary

• Move concrete ID prefixes to the ID type seams instead of passing logical names through Identifier.
• Replace app Identifier.ascending(\"message\" | \"part\") calls with local MessageID / PartID constructors.
• Leave Identifier as the low-level monotonic ID implementation that only accepts concrete prefixes.

Testing

• bun test --timeout 5000 test/server/httpapi-query-schema-drift.test.ts test/cli/github-action.test.ts test/project/migrate-global.test.ts test/session/message-v2.test.ts
• bun typecheck from packages/opencode
• bun typecheck from packages/app
• bunx prettier --check packages/opencode/src/id/id.ts packages/opencode/src/session/schema.ts packages/opencode/src/control-plane/schema.ts packages/opencode/src/permission/schema.ts packages/opencode/src/question/schema.ts packages/opencode/src/pty/schema.ts packages/opencode/src/tool/schema.ts packages/opencode/src/sync/schema.ts packages/app/src/utils/id.ts packages/app/src/components/prompt-input/submit.ts packages/app/src/components/prompt-input/build-request-parts.ts packages/app/src/pages/session.tsx
• bunx oxlint ... (warnings only in existing app files, no errors)
• pre-push bun turbo typecheck

Stack

• Source HTTP API ID path patterns #26623
• Source workspace path pattern #26632
• Remove redundant ID Zod overrides #26633
• Localize ID prefixes #26637 ← current

[kitlangton]

Closing this for now. The local-prefix change is a design detour; we can revisit a cleaner shared @opencode-ai/core ID constructor seam in a separate follow-up.