feat(skills): generalize PR readiness for contributors

[sd-db]

Description

The project PR-readiness skill previously assumed an upstream maintainer was preparing another contributor's PR. That excluded contributors checking their own changes and made fork destinations depend on checkout conventions.

This change makes the workflow usable from supported coding agents while keeping writes explicit and narrowly scoped:

• Stores the canonical skill and behavioral contract under .agents/skills, with a thin .claude compatibility entry point.
• Classifies contributor-owned, in-repository, opted-in maintainer, and read-only PR modes from live metadata.
• Resolves the exact head repository and branch, then revalidates identity, permissions, destination, and head SHA before commits and pushes.
• Keeps worktrees and reports under ignored .agents paths and makes the PR-template wording agent-neutral.

Checklist

• I have run this workflow in development and verified its decision behavior
• This PR includes a co-located behavioral contract covering contributor, fork, maintainer, moved-head, and approval-gate cases
• I have updated the CHANGELOG.md — not applicable because this changes contributor workflow documentation, not adapter runtime behavior
• [Optional] I have run the dbt-databricks-pr-ready project skill for this PR and addressed its merge-readiness feedback

Verification

• hatch run pre-commit run --all-files
• Static reference, portability, and forbidden-token scans
• Six-case OLD/NEW decision comparison with no regressions or residuals

[github-actions]

Coverage report

This PR does not seem to contain any modification to coverable code.