feat: Rules management (browse and edit rules across tools)

[Railly]

Follow-up from #25.

Rules from Cursor (~/.cursor/rules/) and Windsurf (~/.windsurf/rules/, ~/.codeium/windsurf/memories/) are already scanned by the plugin, but there is no dedicated management surface for them.

Scope

• First-class Rules view (or filter) alongside Skills, Commands, and Agents
• Create/edit rules with the same stepped wizard and inline editor used for skills
• Cover per-tool rule formats: Cursor .mdc frontmatter (globs, alwaysApply), Windsurf plain md
• Consider project-level rules (.cursor/rules/ inside scanned projects), not just global

Requested by @AndyM129 in #25.