docs(workflow): move specs under docs and use major action tags

[PerfectPan]

Title format: type(scope): summary

Summary

• specs/ moves to docs/specs/, next to docs/plans/.
• Workflows use actions/checkout@v7 instead of a commit SHA; CONTRIBUTING states that actions are referenced by their latest major tag.

Motivation

• Keeping Specs and Plans together under docs/ puts every design document in one tree.
• The owner prefers major version tags over SHA pins for actions.

Implementation Notes

• Relative links in docs/specs/*.md now point to ../../CONTRIBUTING.md; docs/README.md links specs/ and plans/ as siblings.
• check-repository.sh requires docs/specs/0000-template.md.
• README, AGENTS.md, CONTRIBUTING.md and CHANGELOG updated.

Validation

• Repository checks: ./scripts/check-repository.sh and --staged ok; git diff --cached --check clean.
• grep -rn "specs/" finds no reference to the old root path outside CHANGELOG history entries.
• This PR's Review run uses actions/checkout@v7.

Skipped gates and reasons:

• No stack-specific gates in this template.

Evidence

• Review workflow run on this PR.

Safety Checklist

• No credentials, tokens, private hostnames, personal filesystem paths, or generated logs are included.
• Local config, generated output, build artifacts, and temporary workspaces are not staged.
• User-facing behavior, docs, changelog, migrations, or rollback notes are updated when relevant.
• Completed Spec/Plan constraints are migrated to tests or current-state docs before retirement; unfinished scope remains active.
• The branch is current enough for review, and the remote head matches the intended commit.

Follow-up Risks

• Downstream repositories need to move their specs/ directory and re-sync review.yml.