perf: cache session history with workspace images

[seven332]

Summary

• cache verified final session-history bytes as a sidecar beside promoted workspace images
• probe the sidecar on fresh workspace-image hits and restore locally before falling back to R2
• share sidecar protocol metadata in guest-contracts, cap sidecars at the current 64 MiB copy-file stream limit, and include sidecar bytes in cache GC/inspection accounting

Testing

• cargo test --manifest-path crates/Cargo.toml -p guest-agent session_history_identity -- --nocapture
• cargo test --manifest-path crates/Cargo.toml -p runner session_history_sidecar -- --nocapture
• cargo test --manifest-path crates/Cargo.toml -p runner restore_plan_ -- --nocapture
• cargo test --manifest-path crates/Cargo.toml -p runner run_in_sandbox_restores_session_history_from_workspace_sidecar -- --nocapture
• cargo test --manifest-path crates/Cargo.toml -p runner workspace_promotion::tests -- --nocapture
• cargo check --manifest-path crates/Cargo.toml -p guest-agent -p runner
• cargo clippy --manifest-path crates/Cargo.toml -p guest-agent -p runner --all-targets -- -D warnings

Closes #20732

[codecov]

Codecov Report

❌ Patch coverage is 70.46569% with 241 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/runner/src/executor/sandbox_run.rs	38.38%	61 Missing ⚠️
crates/runner/src/workspace_promotion.rs	63.71%	41 Missing ⚠️
crates/guest-agent/src/main.rs	2.43%	40 Missing ⚠️
crates/runner/src/executor/agent_run.rs	74.41%	33 Missing ⚠️
crates/runner/src/workspace_image_cache/sidecar.rs	82.35%	33 Missing ⚠️
...es/runner/src/executor/session_history_download.rs	75.60%	10 Missing ⚠️
...ates/runner/src/workspace_image_cache/lifecycle.rs	83.60%	10 Missing ⚠️
crates/runner/src/workspace_image_cache/types.rs	0.00%	10 Missing ⚠️
crates/guest-agent/src/session_history_identity.rs	95.34%	2 Missing ⚠️
crates/runner/src/cmd/start/job_discovery.rs	95.65%	1 Missing ⚠️

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[github-actions]

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