Add /document-feature slash command

[andre-crator]

Summary

Adds /document-feature, a slash command that generates paired documentation for an existing feature in a project:

• docs/dev/<slug>-implementation.md — technical write-up: overview, files involved (with paths/line refs), architecture and data flow, API/interfaces, data models, implementation notes, dependencies, and tests.
• docs/user/how-to-<slug>.md — plain-language user guide: what the feature does, step-by-step instructions with [SCREENSHOT: ...] placeholders at each state change, and a troubleshooting section sourced from real error messages in the code.

Key behaviors:

• Normalizes the feature name into a kebab-case slug used consistently across both file names.
• Searches the codebase (Glob/Grep) for the feature before writing anything — if nothing matches, it stops, lists what it searched for, suggests partial matches if any, and asks the user instead of inventing content.
• Detects and follows existing documentation conventions in the repo (docs/dev/, docs/user/, or a style guide) if present.
• Warns before overwriting existing docs at the target paths.

Also updates README.md (previously just a one-line title) with a Commands table and install/usage instructions, so the command is discoverable.

Test plan

• Ran the command against a real feature in a separate project (an expense-tracker export flow) and confirmed it produced accurate, non-hallucinated docs, correctly stopped and asked when pointed at a feature that didn't exist yet, and warned before overwriting existing docs.