feat(progress): add agent-specific emojis and labels to progress update headers

[aaight]

Summary

• Add AGENT_LABELS map and getAgentLabel() helper to src/config/agentMessages.ts with agent-specific emojis and labels (e.g., implementation → 🧑‍💻 Implementation Update, review → 🔍 Code Review Update)
• Update PROGRESS_SYSTEM_PROMPT and formatProgressUserPrompt() in src/backends/progressModel.ts to inject the agent-specific header into the LLM prompt context
• Update formatStatusMessage() in src/config/statusUpdateConfig.ts to use the new getAgentLabel() helper instead of the generic "**I'm making progress**" header
• Add tests for getAgentLabel() (all 9 known agents + unknown fallback) and update existing assertions to match new headers

Card: https://trello.com/c/tAGmOAKD/87-where-we-say-progress-update-1-min-lets-make-this-header-more-engaging-and-also-signify-which-agent-is-running-for-example-%F0%9F%A7%91%F0%9F%92%BB-im

Test plan

• All 2716 unit tests pass
• Type checking passes (zero errors)
• Lint passes (zero errors)
• New getAgentLabel() tests cover all 9 agent types + unknown fallback
• formatStatusMessage() assertions updated to verify new agent-specific headers
• Manual: progress comments will now show 🧑‍💻 Implementation Update, 🔍 Code Review Update, etc. instead of generic I'm making progress

🤖 Generated with Claude Code

[nhopeatall]

🔍 Reviewing PR...

[nhopeatall]

LGTM — Clean, focused change that replaces the generic "I'm making progress" header with agent-specific emojis and labels.

The implementation is well-structured: AGENT_LABELS map and getAgentLabel() are correctly co-located with the existing INITIAL_MESSAGES map in agentMessages.ts, both consumers (progressModel.ts for LLM prompt context, statusUpdateConfig.ts for template fallback) are updated consistently, and the agent type keys match exactly between AGENT_LABELS and INITIAL_MESSAGES.

All CI checks pass. Tests cover all 9 agent types plus the unknown fallback. No issues found.