feat(skills): release-readiness-gate skill + release-readiness collection

## Skill Name

release-readiness-gate (+ new `release-readiness` collection)

## Programming Language

Other — documentation-driven playbook skill (no executable scripts)

## Purpose

Provides a go/no-go **release gate** that consolidates the specialist planner outputs (RAI, Security, Supply Chain, Performance, Privacy, Accessibility) into a single, evidence-grounded ship decision — **Go / Conditional-Go / No-Go** — with a RAG scorecard per readiness pillar, a blocking-gap list, and a sign-off checklist, scored against a PRD trust bar or a default pillar set. This is the TPM-facing artifact presented at a launch review; it decides, it does not remediate.

## Prerequisites

None — documentation-driven playbook. Consumes existing planner artifacts under `.copilot-tracking/` (`rai-plans/`, `security-plans/`, `sssc-plans/`, `performance-plans/`, `privacy-plans/`, `accessibility/`).

## Requirements

- Standalone `user-invocable` playbook skill (Procedure + Verdict Rules + Output Format); no agent/state machine.
- RAG scorecard per pillar with an Evidence-source mapping to the producing planner; pillars without a dedicated planner (Observability, Operational Readiness) are assessed from codebase signals.
- Verdict rules (No-Go / Conditional-Go / Go) and a Handoff that routes blocking gaps back to the owning specialist planner.
- Output to `.copilot-tracking/release-readiness/--readiness.md`, governed by the `disclaimer-language` and `untrusted-content-boundary` instruction globs.
- New `release-readiness` collection bundling the gate skill plus the shared governance instructions (`disclaimer-language`, `untrusted-content-boundary`, `hve-core-location`) for isolated-install portability.
- Registered in the `hve-core-all` bundle.