fix: resolve 'me' to current user in all commands

[steveclarke]

Summary

• Adds "me" → current user resolution in UserResolvable#resolve_user, fixing all 9 commands that accept a user argument: cards assign, cards unassign, cards assigned, cards create --owner, spaces add-member, spaces remove-member, activity --user, projects create --owner, projects update --owner
• Case-insensitive matching (me, Me, ME all work)
• Cached via @me_cache (consistent with existing @users_cache pattern)
• Single change in the shared concern — no per-command fixes needed
• Updated --help desc strings, README tip, and CLI skill docs to reflect me as valid everywhere

Fixes #77

Readiness

• Simplify: clean (guard clause style, added caching)
• Code review: clean (0 critical, 2 auto-fixed)
• Finalize: tests pass (745/0), lint clean

Test plan

• New unit tests for UserResolvable#resolve_user (7 examples covering nil, ID passthrough, name resolution, not-found error, and me/Me/ME)
• Updated activity integration tests to stub /users/me endpoint
• Full suite passes (745 examples, 0 failures)
• Manual: suth cards assign CARD me assigns current user
• Manual: suth cards unassign CARD me unassigns current user

[steveclarke]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

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