refactor(message)!: expose spawned task handles

[Kilerd]

Messager::spawn previously accepted only unit-output messages and discarded Tokio's task handle. It now returns JoinHandle<M::Output>, allowing callers to await values or application errors, observe panics/cancellation through JoinError, and abort work. The implementation returns the handle from the existing tokio::spawn call without adding another task or allocation.

Keep Message / Messager as an optional typed-command convention with application context. Document that send executes in the caller's future, while spawned tasks require caller-managed lifetimes. Update the README, compile-checked API example, runnable message example, and migration guide to match.

Compatibility: calls with a semicolon still compile; code expecting spawn to return () must adapt. Dropping a handle still detaches its task, including when the owning future is cancelled. Spawned messages are not automatically joined by application shutdown; awaiting send inside an owned scheduled execution keeps that work within the scheduler's existing lifetime.

Validation:

• Four focused regression tests cover result/error and panic propagation, lazy inline execution and cancellation without a runtime, abort-and-join, and detach-on-drop; the existing context test is retained.
• cargo test -p gotcha and cargo test -p gotcha --all-features (including doctests).
• cargo check -p message.
• cargo clippy --all-features --workspace -- -D warnings.
• RUSTDOCFLAGS='-D warnings' cargo doc -p gotcha --all-features --no-deps.
• cargo fmt --all --check and git diff --check.

All 11 GitHub CI checks passed for d7bf36f7e84a5086d8b869451507c1b1b5a9cf27.

Closes #96.