docs: add task signing command examples

[haileymarshall]

Summary

• Add a command reference for generating validation files.
• Document L2 gas estimation flags and a deposit-transaction example.
• Add task-origin signing and verification examples.
• Document the repository checks contributors should run.

Why

Fixes #167. The README described the tool's concepts but did not provide practical command examples, flag explanations, or sample workflows for the validation and task-origin scripts.

Testing

• git diff --check
• Documentation-only change; no application code changed.
• Commit signed with a verified SSH signing key.

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1