fix(webview): open terminal file links in editor

[islee23520]

Summary

• parse terminal file references including @src/foo.ts:12:3, src/foo.ts, src/foo.ts:12, README.md#L10, and file:///... paths
• route file link activation through the existing openFile webview message
• extract host-side open-file handling and reject invalid line/column and non-file URL payloads before touching the editor

ULW plan

• From plans/triaged-issues-ulw-plan.md, Wave 3 (Can I open the file via ctrl + click the file in opencode sessions? #42 file link Ctrl/Cmd-click open)

Verification

• RED: npm run test -- src/webview/links/index.test.ts src/providers/MessageRouter.test.ts src/types.test.ts failed before implementation for at-prefixed path suffix parsing, file URI suffix parsing, and invalid openFile location guards
• GREEN: npm run test -- src/webview/links/index.test.ts src/providers/MessageRouter.test.ts src/types.test.ts (74 tests)
• GREEN: npm run compile
• GREEN: npm run lint -- src/webview/links/index.ts src/webview/links/index.test.ts src/providers/openFile.ts src/providers/MessageRouter.ts src/providers/MessageRouter.test.ts src/types.ts src/types.test.ts
• GREEN: npm run compile:e2e
• GREEN: git diff --check origin/main...HEAD
• Manual QA channel: tmux-run targeted file-link/openFile tests passed (QA_DONE:0)

Note

MessageRouter.ts is still larger than ideal, but this change extracts open-file behavior to src/providers/openFile.ts instead of adding more logic to the router.