Add configurable sort policy for GetReadyWork (bd-147)

- Add SortPolicy type with hybrid, priority, oldest constants
- Add SortPolicy field to WorkFilter
- Implement buildOrderByClause() for SQL generation
- Add --sort flag to bd ready command
- Add comprehensive tests for all 3 sort policies
- Update RPC protocol to support sort policy
- Update documentation with sort policy examples

Enables autonomous systems like VC to use strict priority ordering
while preserving hybrid behavior for interactive use.

Amp-Thread-ID: https://ampcode.com/threads/T-9d7ea9db-8d6d-4498-9daa-48a7e104ce1f
Co-authored-by: Amp <amp@ampcode.com>

README.md

@@ -237,6 +237,11 @@ bd ready --limit 20
 bd ready --priority 1
 bd ready --assignee alice
 
+# Sort policies (hybrid is default)
+bd ready --sort priority    # Strict priority order (P0, P1, P2, P3)
+bd ready --sort oldest      # Oldest issues first (backlog clearing)
+bd ready --sort hybrid      # Recent by priority, old by age (default)
+
 # Show blocked issues
 bd blocked