feat: add OpenCode V2 support

[alexhawat]

Summary

Adds OpenCode V2 support alongside the existing V1 plugin, in one package.

• V1 keeps the named OtelPlugin export, unchanged.
• V2 reads a new default export (id: devtheops.otel, setup()), built on V2's granular
  event stream.

Closes #128.

Why the handlers were re-architected

The V1 handlers consume events that no longer exist in V2 (session.idle,
message.updated, message.part.updated, permission.*, command.executed, session.diff).
V2 replaced them with a granular taxonomy, so the handlers were rewritten rather than adapted.

What V2 emits

Signal	V2 source
opencode.session run span (AGENT, turn root)	prompt hook → session.execution.succeeded/failed/interrupted
Session count + session.created log	session.created, or lazily on first sight
session.duration, session.token.total, session.cost.total	session.usage.updated, turn end
opencode.llm step span (LLM) + token/cost/cache/model metrics	session.step.started/ended/failed
opencode.tool.<name> span (TOOL) + tool.duration	session.tool.input.started, called, success/failed
retry.count	session.status (retry)
user_prompt log (linked to the run span)	first step of each turn
session.error log + error span status	session.execution.failed
W3C trace-context header injection	model.request hook

Parity gaps

Not ported (no V2 equivalent): message/part spans, permission telemetry, command.executed,
and session.diff (lines-of-code/commit) metrics.

Shared changes (V1 + V2)

• src/config.ts: additive redactSecrets / redactValues options (V1 ignores them).
• src/headers.ts: replace the Bun global with node:child_process so the headers helper is
  runtime-agnostic; the "terminated by SIGTERM" error is preserved.
• src/redact.ts: new, best-effort secret masking.

V2 specifics worth knowing

• V2 loads a global plugin per location and delivers every event to every instance. The
  V2 entry keeps its OTel providers and tracing state on globalThis (one shared instance,
  flushed but never shut down) and dedupes events by event.id.
• V2 does not emit session.idle, and session.created is absent for pre-existing sessions,
  so the turn root span is anchored on session.execution.*.
• session.tool.called/success/failed carry no tool name; it is read from
  session.tool.input.started.data.name and correlated by call id.

Testing

bun run lint, bun run check:jsdoc-coverage (87.96%), bun run typecheck, and bun test
(331 pass) are all green locally. Added tests/redact.test.ts. Verified end-to-end against
OpenCode 2.0.15 with live OTLP export (spans/logs/metrics).

Notes for review

• src/v2/util.ts is a copy of src/util.ts retyped against the V2 context. Happy to refactor
  to a shared generic util if you prefer.
• src/v2/ is self-contained and does not touch the V1 files (only src/index.ts gains one
  re-export line), so V1 behaviour is unchanged.

[alexhawat]

Follow-up commit: fix(v2): nest subagent traces under the parent dispatch tool span

Testing against a real subagent surfaced two problems in the initial V2 port:

• Subagent turns produced an orphan root run span, because the run span was created on the prompt hook (which fires before session.created is processed) and always used the root context.
• The subagent's session span was only ended on session.idle — which V2 never emits — so LLM/tool spans pointed at a parent span that was never exported.

Changes:

• Create the run span on session.execution.started (event-ordered after session.created), not the prompt hook.
• Nest subagent run spans under their session span, marked session.is_subagent.
• Parent the subagent session span to the parent's opencode.tool.subagent span.
• End subagent session spans on execution end so they export.
• Prefer the active run span when resolving LLM/tool parents.

Resulting tree (one trace per execution):

opencode.session (parent run)
  └─ opencode.tool.subagent
       └─ opencode.session (subagent session, is_subagent=true)
            └─ opencode.session (subagent run)
                 ├─ opencode.llm
                 ├─ opencode.tool.read
                 └─ opencode.tool.shell

Verified against OpenCode 2.0.15 with a real subagent dispatch; bun run lint, check:jsdoc-coverage (87.96%), typecheck, and bun test (331 pass) are green.