feat(wiki-research): add optional pluggable CLI retrieval backends

[Ar9av]

Summary

• Adds an optional vault file references/research-backends.md that registers extra CLI retrieval backends (name, binary, invoke template, when-to-use, cost tier, env key)
• In Rounds 1–2, the skill checks active backends against each angle/URL and folds their output into the same claims/concepts/entities/contradictions extraction
• Ships a starter YAML registry template (defuddle, yt-dlp, perplexity-sonar) inline in the skill

Key guarantees

• Zero regression: no research-backends.md → behaviour identical to today
• Graceful fallback: missing binary / unset env key / non-zero exit → backend skipped with a note; research continues via WebSearch/WebFetch. A backend never aborts a run.
• Free-first ordering: free backends evaluated before paid ones per angle
• CLI hooks, not MCP: same pattern as existing defuddle hook; no server wiring, no hard dependency

Net change

+78 lines in .skills/wiki-research/SKILL.md, fully additive and optional.

Test plan

• Vault with no research-backends.md → skill runs identically to before
• Vault with research-backends.md containing an unavailable binary → backend noted as skipped, research completes normally
• Vault with a paid backend whose env_key is unset → backend skipped, noted in summary
• Vault with a working free backend (e.g., defuddle) → invoked for matching URLs, output folded into extraction
• Backend that exits non-zero → skipped for that angle, run continues

Closes #106

[Aspyder1]

Reviewed: scoped, additive-only (opt-in via references/research-backends.md, zero regression when absent), graceful fallback on missing binaries/env keys, CI green (install-leaves-tree-clean passing), closes #106. Looks ready to merge.