Detect opencode as an AI agent in GetAIAgent() #664

Problem

opencode (the open-source AI coding agent) sets two environment variables unconditionally at startup for every session:

• process.env.AGENT = "1"
• process.env.OPENCODE = "1"

Source (opencode, pinned to commit 3f39a329c3d52ed66405c4bc6293b9ed08fe9ab6):
https://github.com/anomalyco/opencode/blob/3f39a329c3d52ed66405c4bc6293b9ed08fe9ab6/packages/opencode/src/index.ts#L75-L76

Because AGENT is set to the literal "1", the generic fallback in GetAIAgent() catches it and records the agent name verbatim as "1". As a result, opencode-driven CLI runs surface in the User-Agent header, the ai_agent telemetry field, and the Jaeger ai_agent trace tag as:

slack-cli/<version> (os: darwin) AI-Agent (name: 1)

So the run is detectable as an AI agent but mislabeled as 1 instead of opencode, which is noisy for agent attribution / dashboards.

Note on sourcing: opencode's published docs (opencode.ai/docs) do not document these env vars; the authoritative source is the repo source code linked above.

Proposed change

Add a dedicated OPENCODE case to GetAIAgent() in internal/useragent/useragent.go, matching the existing hardcoded agents (claude-code, codex, gemini-cli, cline, cursor). It must be placed before the generic AGENT fallback so opencode wins over the bare AGENT=1:

case os.Getenv("CURSOR_AGENT") == "1":
    return &AIAgent{Name: "cursor"}
case os.Getenv("OPENCODE") == "1":
    return &AIAgent{Name: "opencode"}
case os.Getenv("AGENT") != "":
    return &AIAgent{Name: os.Getenv("AGENT")}

The GetAIAgent doc-comment priority list should also be updated to include OPENCODE (e.g. ... > CURSOR_AGENT > OPENCODE > AGENT).

Offer

Happy to send a PR with this change plus a table-driven test case in useragent_test.go, pending maintainer feedback per CONTRIBUTING.md.