feat(cli): friendly error when record/watch runs outside a Cargo project #70

Running cargo-chronoscope record (or watch) in a directory without a Cargo.toml currently surfaces cargo's raw error through the supervisor pipeline, which is noisy. The user gets:

error: could not find `Cargo.toml` in `/some/path` or any parent directory

— wrapped in JSON event-stream warnings that are hard to parse for someone who just typo'd a cd.

Proposal

Detect the missing manifest before spawning cargo and exit with a clean error:

error: no Cargo.toml in /some/path or any parent directory.

cargo-chronoscope must be run from inside a Rust project (the same place `cargo build` would work). Try:
    cd /path/to/your/rust/project
    cargo-chronoscope record

Walk up from workspace_dir looking for Cargo.toml, identical to what cargo does. If none found, return a friendly anyhow! error from cmd_record / cmd_watch before reaching supervisor::spawn_build.

Acceptance

• In a directory with no Cargo.toml (and no parent with one), cargo-chronoscope record exits non-zero with the friendly message above.
• In a workspace root (Cargo.toml exists), behavior is unchanged.
• In a workspace member (nested directory whose parent has Cargo.toml), behavior is unchanged.
• Existing tests still pass; one new test covers the no-Cargo.toml case.

Files

• src/main.rs — cmd_record, cmd_watch get a pre-flight check, or a small helper
• tests/ — new integration test using tempfile::TempDir