feat: implement card assign/unassign commands and edit flags

[brianevanmiller]

Summary

Complete the stubbed card assign and card unassign commands so users can manage card assignees from the CLI. Also add --assign/--unassign flags to card edit for non-interactive use.

• bc4 card assign — Accepts positional args (@jane @john) and/or --assign flag, resolves users via UserResolver, merges with existing assignees (deduplicates)
• bc4 card unassign — Same input flexibility, filters resolved users from existing assignees, handles edge cases (no assignees, users not assigned)
• bc4 card edit --assign/--unassign — Adds --assign and --unassign string slice flags to the non-interactive mode, extending updateCardNonInteractive with merge/filter logic
• Fix parseSince for "this week"/"last week" — Truncate to midnight (matching "today"/"yesterday" pattern), fixing a test that failed on Sundays

bc4 card assign 12345 @jane @john
bc4 card assign 12345 --assign user@example.com
bc4 card unassign 12345 @jane
bc4 card edit 12345 --assign @jane --unassign old@example.com

Test plan

• make build succeeds
• go vet ./... passes
• go fmt ./... clean
• go test ./... — all tests pass (including previously failing TestParseSince/this_week)
• bc4 card assign --help, bc4 card unassign --help, bc4 card edit --help show correct flags/examples
• Manual test: assign a user to a card, verify in Basecamp UI
• Manual test: unassign a user, verify removal in Basecamp UI
• Manual test: bc4 card edit ID --assign @user works in non-interactive mode

🤖 Generated with Claude Code

[brianevanmiller]

@brigleb - give this a lookover when you get a chance, thank you!