fix(gnd): surface anyhow error chains in error output

[mvanhorn]

Summary

gnd error output now surfaces the full anyhow error chain instead of only the top-level message. The bare {} Display formattings in main.rs and watch.rs printed just the outermost context (for example failed to load manifest), hiding the underlying cause a developer needs to act on; they now use the alternate {:#} chain rendering. The two bare serde_yaml::from_str()? sites in commands/build.rs and manifest.rs gain context so YAML parse failures name the file being parsed.

Why this matters

Issue #6360 reports that gnd swallows error causes, leaving developers with messages like Error: failed to start watch mode and no underlying reason. The silenced chains were verified at main.rs:266 and watch.rs:49/104 before the change. This covers the error-display half of the issue; the issue also discusses restructuring some error sites to attach more context at creation time, which is a larger refactor left for a follow-up.

Testing

A new unit test asserts a manifest parse failure renders with its cause chain (file name + YAML error) rather than the bare context string. cargo fmt --check and cargo check -p gnd pass; the new manifest error-chain test passes.

Refs #6360