transcripts_dir uses .expect() for HOME env var — should return an error for this environmental failure

[ooloth]

Why

transcripts_dir() panics with .expect("HOME not set") when the HOME environment variable is absent. A missing environment variable is an environmental failure (not a programmer error), and the project convention documented in CLAUDE.md is to return anyhow errors for such cases rather than panicking.

Current state

workflows/src/implement.rs line 13: std::env::var("HOME").expect("HOME not set"). If HOME is absent, the process panics rather than propagating a structured error. Config::load() elsewhere in the project reads env vars via anyhow context chains, establishing the expected pattern for this class of failure.

Ideal state

• transcripts_dir returns Result<PathBuf> and propagates a missing HOME as anyhow::Error with .context("HOME not set").
• Callers receive a structured error they can propagate or surface with context.

Starting points

• workflows/src/implement.rs — line 13, the std::env::var("HOME").expect(...) call
• config/src/lib.rs — Config::load() for reference on the anyhow error-propagation pattern

QA plan

1. Call transcripts_dir() in a context with HOME unset — expect an Err returned, not a panic.
2. Run cargo test — expect all tests pass.

Done when

transcripts_dir() returns Result<PathBuf> and propagates a missing HOME as a structured error consistent with the rest of the codebase.