feat(skills): bundle log-food Claude Code skill in-repo

[phitoduck]

Summary

Moves the `log-food` Claude Code skill out of phitoduck/private-claude-marketplace and into this repo. The skill lives next to the CLI it drives, so contract changes (CLI surface) and consumer changes (skill prose) ship in the same git history.

What changed

• `skills/log-food/SKILL.md` — full rewrite for the post-overhaul CLI:
  • The old "14-pick Python probe" workflow is gone. Replaced by `loseit describe-food ...`, which batches concurrent fetches and returns labeled per-serving data (`primary_serving`, `cross_class_conversion`, `nutrients_per_serving`) — no more ordinal lookups or trace-level cheats.
  • Uses `-o toon` throughout (40–60% fewer tokens than JSON, see feat(cli): add --output toon for token-efficient LLM piping #27).
  • Standardizes on `--serving-amount N --serving-unit X` (the new flag pair from feat(log): --serving-amount + --serving-unit flags for unit overrides #26/fix(parser): Timestamp 2-token instantiate + raw-token day_key + new unit mappings + history mining #33) — `--grams` is gone, ambiguous `oz` is rejected.
  • Diary verification uses labeled keys (`food_measure_unit`, `nutrients_by_label`) — no manual ordinal decoding.
  • Last-resort debugging ladder documented: `describe-food` → `-o toon diary` → `--log-level trace` (safe, headers/cookies suppressed) → `--log-headers` (treat as sensitive) → `chrome-mcp-server`.
• `.claude-plugin/marketplace.json` + `.claude-plugin/plugin.json` — makes the repo a Claude Code marketplace exposing the `log-food` plugin.
• README "Claude Code skill" section — install via:
  ```bash
  claude plugin marketplace add phitoduck/lose-it
  claude plugin install log-food@lose-it
  ```

Catalog reset

The stale `~/.config/loseit/my-foods.toon` cache (built against the pre-overhaul `--grams` workflow with hardcoded pick indices) was backed up to `my-foods.toon.bak.` outside this PR. The new skill doesn't rely on a catalog at all — `describe-food` does the same inspection job inline and gives correct results on the first try.

Follow-up

Separate PR on `phitoduck/private-claude-marketplace` will remove the now-orphaned skill and update its README to point here.

Test plan

• `uv run ruff check`, `ruff format --check`, `pytest tests/ --ignore=tests/functional` — 155 passing, all gates green
• `.claude-plugin/marketplace.json` validates against the documented schema
• SKILL.md frontmatter has `name`, `description`, `triggers` (matches phitoduck-skills format)