feat: configurable trace tags via config file or LANGFUSE_TAGS

[AdJIa]

Closes #34

What

Adds configurable Langfuse trace tags to every exported span, via:

1. tags array in ~/.config/opencode/opencode-langfuse.json (same file as publicKey/secretKey/environment/userId/serviceName)
2. LANGFUSE_TAGS env var, comma-separated (opencode,production)

Both sources are merged and de-duplicated (order-preserving), capped at 50 tags. Empty/whitespace segments are dropped.

Why

In shared Langfuse projects aggregating traces from multiple coding agents (Claude Code, OpenCode, ...), there is currently no way to filter OpenCode traces — the plugin sets no trace tags, while e.g. the Claude Code plugin hardcodes claude-code. This makes per-agent dashboards/queries impossible without manual dist/index.js patching (which every plugin update wipes).

How

• LangfuseCredentialsSchema gains an optional tags: Array<NonEmptyString> (max 50) field — config-file path.
• parseTagsEnv parses LANGFUSE_TAGS for the env-var path.
• Both merge in LangfuseClientService before createLangfuseClient({ ..., tags }).
• makeTraceTagsSpanProcessor(tags) — a new SpanProcessor following the existing makeUserIdSpanProcessor pattern — sets the langfuse.trace.tags OTTL attribute on every span, which @langfuse/core maps to Langfuse trace tags.
• No tags configured → processor not added → zero behavior change for existing users.

Testing

• pnpm run build ✅
• pnpm run lint ✅
• pnpm run format:check ✅
• pnpm run test:integration — 18/18 passed ✅
• End-to-end against self-hosted Langfuse v4.15.0 (events_only mode): verified via local OTLP sniffer that exported spans contain langfuse.trace.tags = ["opencode"], and the tag renders in the Langfuse UI trace list.

README

Documented the tags config field and LANGFUSE_TAGS env var with examples.

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