chore(agents): add Claude subagents for project workflows

[s3bc40]

Summary

• Adds .claude/agents/ with 6 subagents covering the full project lifecycle
• Each agent is under 60 lines with a precise PROACTIVELY trigger condition
• Conventions derived strictly from .claude/rules/ and existing CI configuration

Agents added

Agent	Trigger
python-quality	Any edit to src/devbrief/ or tests/
rust-quality	Any edit to rust/src/
test-runner	After implementing or fixing any feature
subcommand-impl	User asks to implement a new devbrief <cmd>
release-prep	User asks to bump version or cut a release
cicd-guard	Any edit to .github/workflows/

Test plan

• Review each agent file for line count (≤60 lines)
• Verify trigger conditions are specific and non-overlapping
• Confirm all conventions match .claude/rules/*.md and CLAUDE.md
• Confirm cicd-guard invariants match current ci.yml and release.yml exactly

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