feat: support reference file mentions

[nexxeln]

summary

• keep configured scout references available as subagent mentions while adding @name:/path file mention support for files in configured reference roots
• resolve reference file URLs at prompt submission time and keep normal project file search unpolluted until a reference path prefix is typed
• share reference resolution between agent setup, file search, and prompt reads with path traversal protection and cached reference scans

testing

• bun test test/file/index.test.ts --timeout 20000 from packages/opencode
• bun test test/session/prompt.test.ts --timeout 30000 from packages/opencode
• bun test test/server/httpapi-sdk.test.ts --timeout 30000 from packages/opencode
• bun test test/server/httpapi-bridge.test.ts --timeout 30000 from packages/opencode
• bun test src/components/prompt-input/build-request-parts.test.ts from packages/app
• bun typecheck from packages/app
• pre-push hook: bun turbo typecheck