Add AGENTS selector for new threads

[friuns2]

Summary

• detect project AGENTS..md and global ~/.codex/AGENTS..md files for new-thread instruction selection
• start custom-instruction threads through a neutral cwd so default AGENTS.md is not loaded
• pass the selected project cwd on the first turn and keep UI cwd display mapped to the real folder
• document manual light/dark verification steps

Tests

• pnpm exec vitest run src/api/codexGateway.test.ts
• pnpm exec vue-tsc --noEmit
• git diff --check origin/main...HEAD

Notes

• Rebased by creating a fresh branch from origin/main and cherry-picking the task commits. tests.md conflicts were resolved per repo rule by keeping the current base file, then appending the AGENTS selector test section in a separate commit.

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[friuns2]

Screenshot evidence for the new chat AGENTS dropdown: