feat(plugin): add Claude Code plugin with guided skills, agents, and hooks

[rsmets]

Summary

Adds a local Claude Code plugin (abca-plugin/) that gives developers guided workflows for setting up, deploying, operating, and troubleshooting the ABCA platform, directly from their IDE or terminal.

In response to Issue #17.

• 7 skills: setup, deploy, onboard-repo, submit-task, troubleshoot, abca-status, abca-submit
• 2 agents: cdk-expert (infrastructure work), agent-debugger (task failure investigation)
• 1 hook: SessionStart injects project context into every Claude Code session

How to use

claude --plugin-dir abca-plugin

Area

• docs — guides or design sources (docs/guides/, docs/design/)
• tooling — root mise.toml, scripts, CI workflows

Changes

• Created abca-plugin/ directory with plugin.json manifest
• Added 7 skills as SKILL.md files with trigger descriptions and step-by-step workflows
• Added 2 agents with model/color/example frontmatter per plugin spec
• Added SessionStart hook for automatic project context injection
• Updated README.md with plugin installation, usage, and development docs

Acknowledgment

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of the project license.

[krokoko]

@rsmets are you still testing this ? Or we can switch it to ready for review ?