Add local feature flag system and a feature-flags skill

[GordonBeeming]

Summary

• Adds a typed feature-flag registry (src/featureFlags.ts) with default/override resolution; only user overrides persist, and the Rust backend prunes unknown or retired ids on load.
• Surfaces user-facing preview flags under a new Preview Features settings category, seeded with a gitAttribution sample flag (default off) that reserves the id for that future feature.
• Updates the new-feature planning skill to require a feature-flag recommendation per feature, documents the flag lifecycle in docs/development.md, and adds a feature-flags repo skill covering how to add, gate, remove, and promote flags.

Test plan

• ./run-tests.sh (npm 274 tests, build, bundle budget, smoke, npm audit, cargo fmt/clippy/test/check)
• Toggle the gitAttribution flag in Settings → Preview Features, restart the app, confirm it persists
• Add a bogus flag id to ui-state.json, restart, confirm it is pruned
• Confirm internal-only flags never render in Settings

Closes #7

[gemini-code-assist]

Code Review

This pull request introduces a local feature-flag system allowing larger or riskier features to ship behind a flag before graduating to normal settings or becoming always-on. It adds a typed registry in the frontend, persists overrides in the Rust backend, and provides a 'Preview Features' tab in the Settings UI for user opt-in. A review comment suggests optimizing isFeatureEnabled to perform a direct O(1) lookup instead of a full sanitization and resolution on every check, which avoids unnecessary object allocations on hot paths.

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[Copilot]

Pull request overview

This pull request introduces a local, typed feature-flag system (with persisted user overrides) and surfaces user-toggleable preview flags in Settings, alongside documentation and agent-skill guidance for managing the flag lifecycle.

Changes:

• Adds a typed feature flag registry with override sanitization and default/override resolution (src/featureFlags.ts) plus unit tests.
• Persists feature-flag overrides in UI state (frontend + Rust backend) and prunes unknown/retired ids during backend sanitization.
• Adds a Preview Features Settings category for user-visible preview flags and documents the flag lifecycle + adds/updates agent skills.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/tauri.ts	Extends persisted view settings to include featureFlags override map and seeds defaults.
src/tauri.test.ts	Updates hosted Tauri transport test fixtures to include featureFlags.
src/styles.css	Adds styling for preview feature flag rows in Settings.
src/featureFlags.ts	Introduces the typed registry, sanitization, and resolution helpers for feature flags.
src/featureFlags.test.ts	Adds unit coverage for defaults, overrides, sanitization, and preview visibility filtering.
src/App.tsx	Adds Preview Features settings tab and wires persisted featureFlags overrides into app state + persistence.
src/App.test.tsx	Adds rendered-shell coverage ensuring preview flags render and toggles persist/restore.
src-tauri/src/lib.rs	Persists feature flag overrides, prunes unknown ids on load, and adds backend tests for pruning/round-trip.
docs/development.md	Documents the local feature-flag system, lifecycle, and promotion/removal guidance.
AGENTS.md	Adds guidance to use $feature-flags when operating the feature-flag lifecycle.
.agents/skills/new-feature/SKILL.md	Requires a per-feature feature-flag recommendation in new-feature artifacts/issues.
.agents/skills/feature-flags/SKILL.md	Adds a repo skill with step-by-step procedures for adding/gating/removing/promoting flags.

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