feat: add context budget plugin

[frostming]

Summary

• add a context-budget Bub plugin with a 200k default context limit
• anchor estimates on provider-reported usage and estimate only trailing messages
• reserve 16,384 tokens and inject an idempotent tape.handoff instruction near the limit
• keep the usage baseline in TurnState and fall back to full estimation for fresh state
• register the package in the workspace and document configuration and behavior

Validation

• uv run pytest packages/bub-context-budget/tests
• uv run ruff check packages/bub-context-budget
• uv run ruff format --check packages/bub-context-budget