Clarify synthesis and retry contracts

[raphael]

Summary

• give RetryHint one explicit AllowsRetry authority, including terminal timeout classification
• preserve recoverable failure repair before a requested synthesis-only transition
• clarify how bookkeeping, terminal tools, tool metadata, and synthesis transitions compose
• document the runtime state precedence in the repository contracts

Rationale

A planner can request synthesis after an evidence-producing tool batch, but that intent must not override typed recovery guidance from a failed tool. The previous API exposed retry classifications without one canonical retry predicate, which made callers reinterpret reasons and risked parallel orchestration policies.

The runtime now owns the precedence: recoverable failures repair first; otherwise a requested synthesis transition resumes with SynthesisOnly. Metadata remains inert and consumer-owned.

Validation

• make test
• make lint
• git diff --check