Add agentic workflow context files

[ngelman1]

Prepare the Tekton Chains repository for agentic workflows as part of SRVKP-11815.

• AGENTS.md: minimal agent-oriented context file covering build/test commands, key conventions (single binary, vendored deps, Knative runtime, ConfigMap config, Sigstore ecosystem), architecture, and PR rules
• CLAUDE.md: symlink to AGENTS.md
• .claude/skills/: commit-message and release-notes skills adapted for this project
• .agents/skills: symlink to .claude/skills for agent auto-discovery
• .gitlint: commit message linting config (matches operator pattern)
• Makefile: add make gitlint target
• .gitignore: add missing patterns (coverage.txt, .idea/, .DS_Store, etc.)

agentready score

• 69.8/100 (Silver) — baseline assessment
• Most failures are out of scope

Notes

• AGENTS.md is 95 lines (target was <60), though 60 is really short, and doesnt fit much info
• Skills are adapted from the operator PR, with Chains-specific scopes and examples.

Changes

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• Has Docs included if any changes are user facing
• Has Tests included if any functionality added or changed
• Follows the commit message standard
• Meets the Tekton contributor standards (including
  functionality, content, code)
• Release notes block below has been updated with any user facing changes (API changes, bug fixes, changes requiring upgrade notices or deprecation warnings)
• Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

NONE

[tekton-robot]

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