Expand myron-polish skill with frontmatter and new checklist sections

[jwils]

Updates the myron-polish pre-review skill, distilling recurring themes from recent review history into the checklists.

Commit 1 — frontmatter + structural additions

• Frontmatter (name/description) so the skill surfaces in the skill list and /myron-polish is discoverable.
• Diff against a freshly fetched origin/main (or the PR's base branch) everywhere, never local main — a stale local main silently turns "the diff" into thousands of unrelated files.
• New "Pass 4 — Mechanical sweep" step that greps the diff for themes that recur in new code even after the original comment is resolved: trailing whitespace, no-op churn (lines removed and re-added identically), #: annotations missing the space, _ = casts / respond_to? / alias_method / Struct.new / .select{}.map{}, redundant ElasticGraph:: prefixes inside module ElasticGraph, and hardcoded derived type names.
• script/quick_build as a one-time final gate after the polish loop (too slow to run every iteration), to catch cross-gem fallout that per-gem runs miss — with guidance for distinguishing environmental datastore flakes from real fallout (re-run failures in isolation and compare against the base branch's failure profile), checking the exit code directly instead of piping through tail, and booting the test datastore for integration/acceptance specs.
• Concrete gh api graphql command in the review-feedback step for listing a PR's unresolved review threads.
• New checklist sections: moves & namespace changes (keep moves reviewable as moves), extension mechanics (one strategy at a time, extended hooks, reuse central mechanisms), collections & small churn (keep Set semantics, targeted tests for incidental bug fixes, drop compatibility-shaped params), and gems & dependencies (runtime deps before dev deps, optional extensions stay optional).
• Expanded YARD+RBS and examples/generated-artifacts guidance, and a post-review-feedback reload step in the workflow.
• Fixed a duplicate step number in the workflow (two 6.s → 6./7.).

Commit 2 — items distilled from the last month of reviews

Pulled from Myron's inline comments on PRs #1224, #1231, #1247, #1251, #1252:

Code checklist:

• Prefer polymorphism over is_a?/instance_of? type checks (adapters/schema types are meant to be polymorphic).
• Core gems stay ignorant of extensions — extension-specific code belongs in the extension, not a core gem.
• Fix the root cause across a whole category, not just the reported instance; test at that generality.
• Comment non-obvious "why" (skip cases, surprising structure); link a tracking issue for known limitations/TODOs.

Tests checklist:

• A test must not be satisfiable by a degenerate constant return ({}, [], nil, one hardcoded case).
• Make tests self-contained about details that matter (surface hidden helper setup an assertion depends on).
• Drive behavior through the public API rather than instantiating internal collaborators directly.
• Preserve the full set of assertions when relocating a test; don't substitute a thinner version.