feat(core): add fff-backed search service

[nexxeln]

summary

• add a runtime-aware Search service that uses fff-bun under Bun and falls back to the existing ripgrep path under Node and desktop
• route file search, grep, glob, debug commands, routes, and tool wiring through the new service while keeping exhaustive file listing and tree operations on ripgrep
• wire frecency tracking from first use by recording successful search selections through file reads and update the affected tests

testing

• bun typecheck
• bun test test/tool/glob.test.ts test/tool/grep.test.ts test/tool/read.test.ts test/file/index.test.ts test/file/ripgrep.test.ts test/session/prompt-effect.test.ts test/session/snapshot-tool-race.test.ts