Auto-expand worker context budget

[pmbstyle]

What changed

• treat the 6,000-unit worker system-context budget as a soft target
• automatically expand the effective budget when required prompt sections outgrow it
• retain 2,000 units of headroom for high-priority optional context
• keep a 64,000-unit runtime safety limit
• record the soft budget, hard limit, and expansion decision in the context manifest

Why

Worker startup could fail before inference when required system context grew slightly beyond the fixed 6,000-unit compiler budget. This surfaced infrastructure prompt bookkeeping to the agent and encouraged ineffective template-level workarounds.

Impact

Required worker instructions remain lossless, and normal prompt growth no longer requires an agent or operator to edit worker templates. The reported 6461 > 6000 case now compiles automatically with an effective budget of 8,461.

Validation

• full pytest suite
• focused worker context, compiler, and agent-loop tests
• Ruff
• Black check
• git diff --check