Phase 5: using: node flip, comment/orchestrator, README

[rdhar]

Parent: #568

Retire the composite bash and switch action.yml to using: node24. Considers #535 (combine matrix outputs into a single comment).

Scope

• comment.ts: markdown body assembly + diff symbol parsing (+/-/!/~/#) + summary parsing; clean code-point truncation (Q5).
• orchestrator.ts: sequence all modules (the former post flow + command flow).
• action.yml: using: node24, main: dist/index.js; delete all bash steps; inputs/outputs/branding unchanged (zero breaking changes).
• README rewrite + major version tag (e.g. v14) with migration notes.

Entry criteria

• Phases 1–4 merged and soaked.

Exit criteria

• Full CI matrix (terraform + tofu × all fixtures, plan + apply) green; AWS workflow green; outputs identical; README updated; major tag cut; canary before moving the floating major alias.

Compatibility shim

• None (shims retired). Consumers pinned to @v13 are unaffected.

Complexity / Risk

• M / whole-action regression. Detected by full matrix + canary repo.

Work items

• comment.ts + orchestrator.ts + action.yml flip to using: node24
• README rewrite + release notes + major version tag