fix: filter binary asset paths and numeric segments from concept extraction

[CalebisGross]

Summary

• Expand the skip list in FromPath to filter asset directories (images, icons, fonts, docs), generic noise (bytes, data, cache), and documentation directories
• Add isNumeric filter to drop dimension-like segments (96x96, 512x512)
• Simplify FromEventType using strings.CutPrefix (lint fix)

Before: themes included docs/images/mnemonic.png, bytes, 96x96
After: themes include only mnemonic, favicon, etc.

Test plan

• 5 new test cases for binary asset paths, noise segments, and dimensions
• All existing tests pass (no regressions on code paths)
• make build && make check clean

Fixes #305

🤖 Generated with Claude Code

[CalebisGross]

Not a bug — the watcher works correctly. Investigation revealed:

1. touch produces CHMOD events, which sendEvent() intentionally drops (only Create/Write/Remove/Rename are meaningful)
2. Real file edits (echo, sed, editor saves) produce Write events that flow through the full pipeline
3. The heuristic filter correctly rejects temp files (empty content) while passing real edits
4. context_boost works end-to-end: daemon watcher → activity tracker → /api/v1/activity → MCP sync → retrieval scoring

Verified context_boost: 0.199 in MCP recall after editing internal/agent/retrieval/agent.go.

The initial test used only touch commands, which masked the fact that the pipeline was working. No code change needed.