docs: add workflow improvements design proposal

Proposes ALTER WORKFLOW with graph-aware operations (INSERT AFTER,
INSERT OUTCOME/PATH/BRANCH, DROP, REPLACE ACTIVITY) and workflow
cross-reference tracking in the catalog refs table.

Addresses review feedback:
- Fix merge point algorithm to handle nested splits via recursion
- Add activity addressing rules with positional disambiguation for
  duplicate captions
- Replace DROP PATH by 0-based index with by first activity caption
- Add REPLACE ACTIVITY operation for in-place swaps
- Use activityRef grammar rule and LBRACE/RBRACE lexer tokens
- Clarify workflowActivity reuses CREATE WORKFLOW syntax
- Add DESCRIBE output annotation for activity captions
- Fix author attribution
- Move to docs/11-proposals/

Author: engalar