Add rtk-ai as experimental feature

[ScDor]

solves #163

Summary

• Adds a gated RTK integration flow so RTK can be enabled globally and per backend, with prompts/tool guidance wired into Claude, Codex, and OpenCode sessions.
• Improves RTK runtime reliability in headless/service environments by resolving the RTK binary from RTK_BIN, PATH, and common user bin locations.
• Fixes RTK gain parsing to support current RTK JSON shapes (date/week_start/month, *_tokens fields) and prevents false startup noise when older RTK CLIs do not support --opencode.
• Improves preferences patch UX by using optimistic cache updates (with rollback on error), so toggles apply immediately and no longer appear stuck.

Validation

• Frontend build: bun run build
• Backend build: cargo build --release
• Targeted frontend tests:
  • src/services/preferences.test.ts
  • src/components/preferences/panes/UsagePane.test.tsx
  • src/components/preferences/panes/IntegrationsPane.test.tsx
  • src/components/preferences/panes/ExperimentalPane.test.tsx
• Targeted Rust tests:
  • cargo test parse_rtk_gain_output --release

Notes

• check:all still has pre-existing unrelated lint failures outside this PR’s diff.
• Older RTK CLI versions that lack rtk init --opencode are now handled gracefully (logged as informational skip).

[ScDor]

will reopen after some more testing