#114 Fix search to be cross-board by default

Summary

• search called defaultBoard() which injected the configured default board into every query, silently scoping results to a single board
• This made --tag and --assignee filters appear broken when matching cards existed on other boards
• Fix: only add board_ids[] when --board is explicitly passed

Closes #113

Test plan

• go test ./internal/commands/ -run TestSearch — all 6 existing tests pass
• Manual: fizzy search "" --tag TAG_ID --all returns cards across all boards
• Manual: fizzy search "query" --board BOARD_ID still scopes correctly when explicit

🤖 Generated with Claude Code

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Summary by cubic

Search is now cross-board by default. We removed the implicit default board so results include cards from all boards unless --board is set.

• Bug Fixes
  • Added tests to ensure no implicit board scope and to assert call count before indexing; removed redundant searchBoard assignments in tests.
  • Confirmed --tag and --assignee filters work cross-board and do not inject board_ids[] unless --board is passed.