Add unblocked-context-search-rules skill

[cancelself]

Companion skill for the new context_search_rules MCP tool — RFC at unblocked/unblocked#30711.

What

Adds the unblocked-context-search-rules skill for the upcoming context_search_rules tool — the reranked top-K sibling of context_get_rules (which retrieves the exhaustive filtered set). Same data, different ranker:

Skill	Use when
unblocked-context-get-rules (PR #27)	You want the exhaustive filtered rule set for a path/task/language
unblocked-context-search-rules (this PR)	You want the top-K rules most relevant to the specific code change you're about to do

The skill follows the established CLI-first pattern from unblocked-context-search-code — query+instruction parameters, MCP fallback to context_research with a steering instruction.

Changes

• skills/unblocked-context-search-rules/SKILL.md (new, ~100 lines) — modeled on unblocked-context-search-code's shape. Covers triggering, CLI invocation, when to use vs. context_get_rules, input table with query/instruction guidance, path-scoping nuance, and result interpretation (including the citation reference IDs the search variant surfaces).
• skills/unblocked-tools-guide/SKILL.md — mapping table row (CLI=yes, MCP=no, matching the rest of the search_* family), common-flags entry, routing-table fallback row, see-also list.
• README.md — Available Skills table row.

⚠️ Dependencies / release ordering

• Backend: depends on the MCP tool / CLI subcommand landing in unblocked/unblocked#30711. That PR is currently an RFC skeleton; full wire-up + CLI subcommand follows after design approval.
• Skills repo: independent of #27 (which adds context_get_rules), but both touch tools-guide and README in nearby places — whichever lands second gets a trivial conflict (different rows, no logic). I scoped this PR to only the search-rules additions, so the diff is clean.

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