Add persona cost attribution using Burn SDK 2.x tags

[willwashburn]

Context

Follow-up to AgentWorkforce/burn#373.

AgentWorkforce should provide default-on persona cost attribution without requiring users to install burn separately and without routing launches through burn run. Burn remains domain-neutral; AgentWorkforce writes generic enrichment tags and burn reports on tags.

Blocked by

• Expose SDK 2.x launcher tagging primitives and generic tag summary filters burn#373: expose @relayburn/sdk 2.x pending-stamp primitives and generic burn summary --tag / --group-by-tag reporting.

Desired changes

• Depend on @relayburn/sdk 2.x only. Do not use deprecated TS 1.x burn packages.
• Keep AgentWorkforce as the direct launcher for claude, codex, and opencode; do not use burn run.
• Preserve persona source metadata during resolution so launches can stamp personaSource and, for local personas, enough debug context to identify the source layer.
• Compute personaVersion from the effective resolved persona spec after cascade/extends merge and before runtime input substitution:
  • stable-key canonical JSON
  • SHA-256 full hex hash
  • optional 12-character short form for display only
• Wrap existing harness launch with burn bookkeeping:
  • before spawn, call sdk.writePendingStamp(...) with generic enrichment tags
  • while child runs, periodically call sdk.ingest({ harness })
  • after exit/error, stop polling and run one final ingest
  • preserve the harness exit code exactly
• Stamp at least these tags:
  • agentworkforce=1
  • persona=<id>
  • personaTier=<tier>
  • personaVersion=<full sha256>
  • personaSource=<cwd|user|dir:n|library>
• Add opt-out controls:
  • agentworkforce agent --no-burn <persona>[@tier]
  • AGENTWORKFORCE_BURN=0
• Update docs to show generic burn queries after burn#373 lands, e.g. burn summary --tag persona=code-reviewer and burn summary --group-by-tag persona.

Acceptance criteria

• A normal persona launch writes burn attribution tags by default once burn#373 is available.
• Users do not need a separately installed burn binary for this attribution path.
• --no-burn and AGENTWORKFORCE_BURN=0 skip stamp writing, periodic ingest, final ingest, and burn summary output.
• Periodic/final ingest is attempted on success, nonzero exit, and spawn error; child exit code is preserved.
• Persona hash is stable under object key reordering and changes when effective persona content changes.
• Unit tests cover default tracking, opt-out, tag payload, version hashing, and exit-code preservation.

Notes

The burn side should stay generic. Do not add burn summary --persona; use tag filtering/grouping from burn#373.