docs(config): document budget_tokens per-phase override and add ContextBudget.with_phase helper (#412) (#814)

* docs(config): document budget_tokens per-phase override and add ContextBudget.with_phase helper (#412)

- Document that budget_tokens overrides only the active phase's budget
  and takes precedence over configured ContextBudget in build/build_sync.
- Add ContextBudget.with_phase(phase, tokens) ergonomic helper in config.py.
- Delegate override_phase_budget to the new helper in build_policy.py.
- Add tests verifying per-phase override and immutability.

Closes #412

* docs(config): fix review comments on #814 — qualify Sphinx cross-refs, trim build docstrings, regen api manifest