feat: add output cache system with TTL and diff detection

[bz00qa]

Summary

• cache.rs: File-based output cache with deterministic keys (command + cwd hash), configurable TTL, volatile command detection, and line-level diff between cached/current output
• rtk cache-clear: CLI command to reset the cache
• Tracking integration: New cache_hit column in SQLite — all gain/analytics queries exclude cache hits to keep savings stats accurate
• Config: [cache] section in config.toml with enabled (default: true) and ttl_minutes (default: 5), overridable via RTK_CACHE and RTK_CACHE_TTL env vars

How it works

When proxy -f (from #553) or future commands use the cache:

1. Compute deterministic key from command + working directory
2. Check if cached output exists and is within TTL
3. If cache hit: diff against current output, show only changes
4. If cache miss: store filtered output for next invocation

Volatile commands (git status, ls, find, etc.) are never cached.

Tests

11 new tests covering key generation, volatile detection, diff computation, and truncation.

Test plan

• cargo fmt --all passes
• cargo clippy --all-targets passes (no new warnings)
• cargo test — 898 passed, 11 new cache tests all green
• Manual: rtk cache-clear clears cache directory
• Manual: verify [cache] section appears in rtk config

🤖 Generated with Claude Code

[pszymkowiak]

This PR has merge conflicts with develop. Please rebase:

git fetch upstream develop
git rebase upstream/develop
git push --force-with-lease

Labeling awaiting-changes until conflicts are resolved.

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[aeppling]

Hey

We are cleaning up the codebase and improving the project structure for better onboarding. As part of this effort, PR #826 reorganizes src/ from a flat layout into subfolders.

No logic changes — only file moves and import path updates.

What you need to do

Rebase your branch on develop when receiving this comment:

git fetch origin && git rebase origin/develop

Git detects renames automatically. If you get import conflicts, update the paths:

use crate::git;        // now: use crate::cmds::git::git;
use crate::tracking;   // now: use crate::core::tracking;
use crate::config;     // now: use crate::core::config;
use crate::init;       // now: use crate::hooks::init;
use crate::gain;       // now: use crate::analytics::gain;

Need help rebasing? Tag @aeppling