Docs/plugin failure handling.md covers session level

[Darkvader-ship-it]

Description

Related Issue

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)
• Documentation update
• Refactor / code cleanup
• Performance improvement
• Test / CI improvement

---

CI/Test Behavior Changes

What changed in CI or test behavior?

N/A — no CI/test behavior changes

Migration notes

N/A

--- closes #938

Checklist

• All tests pass locally (cargo test --workspace --all-features)
• Code is formatted (cargo fmt --all -- --check)
• Clippy is clean (cargo clippy --workspace --all-targets --all-features -- -D warnings)
• Commit message follows Conventional Commits
• PR description mentions the related issue(s)
• CI/test behavior changes documented above (or marked N/A)
• If CLI flags/subcommands/help text changed, man pages regenerated (make regen-man) and .1 files committed

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