fix(core): retry transient agent provider failures

[cbusillo]

Summary

• retry transient external agent provider failures with bounded same-model backoff
• expose retry metadata in agent status/result/wait/list responses and progress logs
• document automatic agent retry reporting

Notes

• Retries are scoped to the provider execution call only; worktree setup, prompt assembly, gating, auth/config/missing-command/policy/cancel failures still fail fast.
• Fallback models are not implemented in this PR. The final model is tracked and currently remains the requested model.
• Session-scoped cancel now requires strict ownership, so legacy unowned agents can remain visible without being cancelled by an unrelated session.

Validation

• cargo test -p code-core model_facing_agent_queries_are_session_scoped --lib
• cargo test -p code-core agent_tool --lib
• git diff --check
• ./build-fast.sh

Fixes #206