feat(KB-503): add multi-project CLI support

- Add project context utilities for multi-project operations

- Add project subcommands (list, add, remove, set-default, detect)

- Update CLI argument parsing with --project flag support

- Add multi-project documentation and changeset

.changeset/cli-multi-project.md

@@ -0,0 +1,10 @@
+---
+"@gsxdsm/fusion": minor
+---
+
+Add multi-project CLI commands and --project flag
+
+- New `kb project` subcommand: list, add, remove, show, set-default, detect
+- Global `--project <name>` flag for all task operations
+- Project context resolution: flag → default → auto-detect
+- Cross-project task management without changing directories