✨ Add handle-github-issue skill for standalone Claude Code install

[monkut]

Summary

• New skill askcc/skills/handle-github-issue/SKILL.md that drives the full askcc workflow (prepare / plan / develop / issue-review / pr-review / explore / diagnose / fix-ci / validate) inline from Claude Code — embeds the agent personas from askcc/definitions.py, pre-flight gates (label prefix, decision-label, develop-readiness), and post-flight label transitions. No runtime dependency on the askcc Python package; uses gh and the project's standard test/lint/typecheck commands directly.
• README.md gets a new ## Skills section with a curl one-liner (and a tag-pinned variant) that installs the skill straight into ~/.claude/skills/handle-github-issue/, plus a reciprocal cross-link from the askcc install paragraph.
• Drops the empty askcc/skills/request-askcc/__init__.py — package_files(...).iterdir() does not require it.

Key Flows

flowchart LR
    A[user prompt with action + issue URL] --> B[Common pre-flight]
    B --> C{action}
    C -- prepare/plan/develop/explore/diagnose --> D[adopt persona<br/>edit issue body<br/>post summary comment]
    C -- issue-review --> E[adopt persona<br/>post issue comment]
    C -- pr-review --> F[fetch linked PR<br/>adopt persona<br/>gh pr review]
    C -- fix-ci --> G[auto-detect PR if needed<br/>identify failing run<br/>fix + push + comment]
    C -- validate --> H[print PASS/FAIL report]
    D --> I[Common post-flight:<br/>label swap, project board, verification]
    F --> I
    G --> I

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Test plan

• Install the skill standalone via the README curl one-liner and confirm ~/.claude/skills/handle-github-issue/SKILL.md lands in place.
• Invoke each action from Claude Code with a real issue URL and confirm pre-flight gates and post-flight transitions fire as documented.
• Confirm askcc install still copies both bundled skills after the __init__.py removal.