fix(rust): accept subagent lifecycle hooks

[hackberry-lab]

Summary

• recognize subagentStart and subagentStop as valid callback-only lifecycle hooks in the Rust dispatcher
• return the existing empty pass-through response without logging an unknown-hook warning
• cover both recognized names and retain a negative classification check for unrelated hook names

The public SessionHooks API remains unchanged. Sub-agent lifecycle activity continues to be available through the session event stream; this change only prevents valid CLI callbacks from being misclassified as unknown.

Fixes #2781

Validation

• cargo test --features test-support hooks::tests::dispatch_subagent_lifecycle_hooks_returns_empty -- --exact
• cargo +nightly-2026-04-14 fmt --all -- --config-path .rustfmt.nightly.toml --check
• cargo clippy --all-targets --no-default-features --features test-support,local-runtime,derive -- --no-deps -D warnings -D clippy::unwrap_used -D clippy::disallowed_macros -D clippy::await_holding_invalid_type
• cargo test --no-default-features --features test-support,derive -- --test-threads=4 --nocapture
• RUSTDOCFLAGS="-D warnings" cargo doc --no-deps --no-default-features --features local-runtime,derive

[Copilot]

Copilot review overview

🟢 Approval recommended

The focused change correctly avoids the unknown-hook path while preserving existing behavior for unrelated hooks.

Review effort: Balanced
Findings: None

What changed in this PR

Recognizes valid Rust subagent lifecycle callbacks without changing the public hooks API.

Changes:

• Passes through subagentStart and subagentStop with empty output.
• Tests both lifecycle names and an unrelated name.

File	Description
rust/​src/​hooks.rs	Adds lifecycle-hook classification, dispatch handling, and tests.

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