wc-signal-emitter — validate + persist signals

Implements the persistence side of footballfantasy/context/frameworks/signal-framework.md. Agents communicate through structured signal files; this skill is the gate that keeps those files trustworthy, so downstream agents can read rather than recompute.

Workflow

- [ ] 1. Receive the signal (frontmatter + body) and its declared type
- [ ] 2. Validate frontmatter, type, numeric ranges, source citations
- [ ] 3. If valid → write to signals/YYYY-MM-DD-<type>[-<scope>].md and return the path
- [ ] 4. If invalid → do NOT persist; return the validation errors so the caller can fix and retry

Validation checklist

• Frontmatter present: type, round, date, emitted_by, confidence (0–1), source_urls.
• Type known: one of the registry in signal-framework.md (scout, player-ev, clean-sheet, fixture, ownership, candidate, fitness, offspring, verify, chip-plan, transfer-plan, board). Reject unknown types.
• Numeric ranges: any signal that declares a range is checked against it (probabilities ∈ [0,1]; ownership ∈ [0,100]; xEV ≥ 0; confidence ∈ [0,1]). Out-of-range → reject.
• Citations: every factual claim in the body traces to a source_urls entry or is explicitly manager-provided. A signal asserting a predicted XI / injury / price / ownership with no URL → reject (or force confidence ≤ 0.35 and a "needs confirmation" flag if the caller insists it's a best-effort estimate).
• Confidence sanity: confidence matches the framework bands (unconfirmed load-bearing fact ⇒ ≤0.35).