Add agentic workflow context files

[ngelman1]

Prepare the Tekton CLI repository for agentic workflows as part of SRVKP-11822.

• AGENTS.md: Agent-oriented context file covering build/test commands, key conventions (Cobra command tree, golden-file tests, vendored deps, version injection via ldflags), architecture overview, PR conventions, and skill triggers

• CLAUDE.md: Redirect to AGENTS.md (Claude Code convention)

• .claude/skills/: Commit-message and release-notes skills adapted from tektoncd/operator PR #3387, with all review feedback incorporated:

• Makefile: Add PKG variable support to lint-go and test-unit targets to allow running tests on a single package

Agentready Overall Score: 63.3/100 (Silver)

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes tests (if functionality changed/added)
• Run the code checkers with make check
• Regenerate the manpages, docs and go formatting with make generated
• Commit messages follow commit message best practices

See the contribution guide
for more details.

Release Notes

[tekton-robot]

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