Add auto-detect and adaptive noise exclusions (tier 2 + 3)

[CalebisGross]

Summary

• Tier 2: Auto-detect noisy apps at startup — Scans ~/.config/ and ~/.local/share/ (Linux) or ~/Library/ (macOS) for ~30 known noisy app directories (Chrome, VS Code, Slack, Discord, etc). Detected patterns are merged with config exclusions before watcher creation. Closes Auto-detect noisy app directories at first startup #27.
• Tier 3: Adaptive learning from rejections — Tracks filesystem events rejected by the heuristic filter per path prefix. When a prefix hits 50 rejections within 1 hour, it's automatically promoted to a live watcher exclusion. Learned exclusions persist to ~/.mnemonic/learned-exclusions.txt and reload on restart. Closes Adaptive watcher exclusions: learn from repeated heuristic rejections #28.

Together with the static config defaults (tier 1, PR #26), this gives the system a 3-tier intelligent noise suppression pipeline that works out of the box for new users and adapts to each machine over time.

New files

• internal/watcher/filesystem/autodetect.go — App registry + detection logic
• internal/watcher/filesystem/autodetect_test.go — 3 tests
• internal/agent/perception/rejection_tracker.go — Adaptive learning core
• internal/agent/perception/rejection_tracker_test.go — 4 tests (prefix extraction, promotion threshold, cap, persistence)

Key changes

• internal/watcher/watcher.go — Added ExcludableWatcher interface
• internal/watcher/filesystem/watcher_other.go — AddExclusion() + RLock on exclude pattern reads
• internal/watcher/filesystem/watcher_darwin.go — Same as above for macOS
• internal/agent/perception/agent.go — promoteExclusion method, rejection tracking on heuristic failures, load persisted exclusions at startup
• internal/config/config.go — LearnedExclusionsPath field with default ~/.mnemonic/learned-exclusions.txt
• cmd/mnemonic/main.go — Wire auto-detect + learned exclusions path

Test plan

• make test passes (all existing + 7 new tests)
• make check passes (go fmt + go vet)
• make build succeeds
• Manual: Run daemon, verify auto-detected exclusions in startup logs
• Manual: Verify learned exclusions file created after sustained noise from a new source

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