chore: repoint pr-review skill to agents-shared capability

[eskenazit]

Related Issue

Refs naftiko/shipyard#23

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What does this PR do?

Repoints the pr-review skill discovery from the hardcoded framework repo path to the
new agents-shared capability hosted in code-standards.

Three changes:

• AGENTS.md — PR Review Skill section replaced: drops the old framework/.agents/
  path reference and TODO; adds a four-line pointer to the capability + instruction shim.
  Also clarifies issue-first wording ("ensure a tracking issue exists" rather than "open
  an issue before every change").
• .gitignore — adds .agents/skills/ so on-demand synced skill copies are never
  accidentally committed.
• .github/instructions/agents-shared.instructions.md — discovery shim (copied from
  code-standards/agents-shared/instructions/). VS Code agents load it on trigger keywords
  (review PR, fix PR comments, …), check for a local copy of the skill, sync it from the
  Skill Server if missing, then follow SKILL.md.

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Tests

No production code changed. End-to-end validated manually: VS Code agent discovered the
instruction, synced the pr-review skill from the local Skill Server (port 9700), and
executed pr-context.ps1 -Pr 33 successfully.

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Checklist

• CI is green (build + tests)
• Rebased on latest main
• Small and focused — one concern per PR
• Commit messages follow Conventional Commits
• Apache 2.0 license header present on any new Java file (no new Java files)

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Agent Context (optional)

agent_name: GitHub Copilot
llm: Claude Sonnet 4.6
tool: VS Code Copilot Chat
confidence: high
source_event: naftiko/shipyard#23
discovery_method: user_report
files_suspected:
  - AGENTS.md
  - .gitignore
  - .github/instructions/agents-shared.instructions.md