feat(session): persist retry reports in session end

[Gongyl01]

Description

Retry reports are currently available only in the CLI output. They are frozen after Agent.Run returns, but session_end is written inside Agent.Run, so the report is unavailable when the session is finalized.

This PR persists the existing retry report in session_end.llm_retry_report:

• passes the per-run RetryCollector to the review agent;
• freezes the report before session finalization on every terminal path;
• waits for background requests before freezing on the normal path;
• stores the optional report in SessionHistory and embeds it in session_end;
• keeps the CLI's existing post-run Freeze and output path unchanged;
• omits llm_retry_report for clean runs with nothing to report;
• suppresses invalid reports without changing warnings, terminal state, or exit code.

RetryCollector.Freeze remains a deterministic read. The agent and CLI take independent snapshots with the same run ID, and tests verify that their serialized values are equal.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactoring (no functional changes)
• Documentation update
• CI / Build / Tooling

How Has This Been Tested?

• make test passes locally
• Manual testing (describe below)

Additional verification:

• make check
• full test suite with race detection
• retry report is embedded in session_end
• absent reports are omitted rather than serialized as empty objects
• persisted and CLI-equivalent snapshots are fully equal
• invalid reports are suppressed without entering Run errors or agent warnings
• nil collector paths remain unchanged

Checklist

• My code follows the project's coding style (go fmt, go vet)
• I have performed a self-review of my code
• I have added tests that prove my fix is effective or my feature works
• New and existing unit tests pass locally with my changes
• Documentation is not required because this reuses the existing retry report schema

[github-actions]

✅ OpenCodeReview: Review complete: 0 finding(s) across 6 selected item(s).