feat: detect OpenCode as an AI agent

[mwbrooks]

Changelog

The CLI now reports opencode as the AI agent name when run from OpenCode, instead of the literal 1.

Summary

This pull request teaches the CLI to detect OpenCode so its telemetry and User-Agent carry a real agent name.

• OpenCode sets AGENT=1 (not AGENT=opencode), so the AGENT fallback in internal/useragent/useragent.go passed 1 straight through as the agent name.
• It also sets OPENCODE=1 unconditionally, alongside AGENT=1 and OPENCODE_PID, in packages/opencode/src/index.ts - a reliable discriminator present in every process it spawns.
• Adds an OPENCODE case to GetAIAgent() just ahead of the AGENT fallback. Ordering matters: OpenCode sets both variables, so OPENCODE has to be checked first.
• Higher-priority agents still win, since someone can run another agent from inside an OpenCode shell.

internal/tracking/tracking.go already feeds useragent.GetAIAgentName() into the ai_agent telemetry field, so no other production code changes were needed.

Closes #664

Preview

Verified via ./bin/slack version --skip-update --verbose:

Environment	Reported name
OPENCODE=1 AGENT=1	AI-Agent (name: opencode)
AGENT=1	AI-Agent (name: 1) (fallback unchanged)
AGENT=goose	AI-Agent (name: goose) (fallback unchanged)
CURSOR_AGENT=1 OPENCODE=1	AI-Agent (name: cursor) (priority holds)
no agent variables	no AI-Agent suffix

Testing

1. Build the CLI with make build.
2. Run env -u CLAUDECODE -u CLAUDE_CODE_ENTRYPOINT OPENCODE=1 AGENT=1 ./bin/slack version --skip-update --verbose and confirm the user_agent debug line ends with AI-Agent (name: opencode).
3. Re-run with only AGENT=1 set and confirm the name is still 1, so the generic fallback is unchanged for other agents.
4. Re-run with CURSOR_AGENT=1 OPENCODE=1 and confirm cursor wins, so detection priority is preserved.

Note: unset CLAUDECODE/CLAUDE_CODE_ENTRYPOINT when testing from inside Claude Code, otherwise it takes priority and masks the result.

Notes

• OpenCode does not document these variables, so this could break if they rename OPENCODE. The AGENT fallback would then resume reporting 1, which is the current behavior - no worse than today.

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.19%. Comparing base (f36b480) to head (c627192).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #665      +/-   ##
==========================================
- Coverage   78.20%   78.19%   -0.01%     
==========================================
  Files         239      239              
  Lines       18135    18136       +1     
==========================================
- Hits        14183    14182       -1     
- Misses       3952     3954       +2     

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[zimeg]

💾 praise: Thanks for adding support for more agents!

[mwbrooks]

Thanks for the quick review @zimeg! 🙇🏻 🚢