Expose SDK 2.x launcher tagging primitives and generic tag summary filters

[willwashburn]

Context

AgentWorkforce wants default-on persona cost attribution without requiring users to install the burn binary and without routing launches through burn run.

Burn should stay domain-neutral. AgentWorkforce will stamp persona metadata as generic enrichment tags; burn should only understand tags/enrichment, not personas as a first-class concept.

Desired changes

• Expose a launcher-safe pending-stamp API from @relayburn/sdk 2.x / the Rust NAPI binding, wrapping the Rust write_pending_stamp primitive.
• Add the JS facade export and .d.ts types, e.g. writePendingStamp({ harness, cwd, enrichment, sessionDirHint?, spawnStartTs?, spawnerPid? }).
• Ensure the supported harness set covers AgentWorkforce direct launches. Codex and OpenCode already fit pending stamps; Claude needs either documented exact-session stamping or pending-stamp support if a session id cannot be preallocated reliably.
• Expose generic enrichment reporting in burn summary:
  • --tag key=value, repeatable; all provided tags must match folded stamp enrichment.
  • --group-by-tag key, grouping cost/tokens by enrichment value.
  • JSON output should include the grouped tag key/value.
• Do not add persona-specific flags such as --persona; personas are just tags from burn perspective.
• Deprecate or remove burn run from preferred docs/help so new integrations use SDK primitives instead of the process wrapper.

Acceptance criteria

• @relayburn/sdk 2.x exports and documents writePendingStamp with typed options/results.
• A Node caller can write a pending stamp, run ingest, and see enrichment folded onto matching turns.
• burn summary --tag persona=code-reviewer filters by folded enrichment without any persona-specific code path.
• burn summary --group-by-tag persona groups usage/cost by generic tag value.
• Rust CLI, SDK, and NAPI tests cover the new tag filter/grouping and pending-stamp export.
• Docs make clear that burn run is legacy/deprecated or no longer preferred for new launchers.

Downstream

Once this lands, AgentWorkforce can depend on @relayburn/sdk 2.x directly and wrap persona launches with generic burn tags: persona, personaTier, personaVersion, personaSource, and agentworkforce=1.