docs: document Go support policy (latest two releases)

Updates README and CONTRIBUTING to reflect current Go version support:
- README: Update minimum Go version from 1.21+ to 1.24+
- README: Document that we support and test the latest two releases (1.24, 1.25)
- CONTRIBUTING: Update CI section to reflect Go 1.24 and 1.25 testing

This aligns documentation with the actual CI testing implemented in PR #11.

Resolves beads-oss-tasks-8vz

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: timvw

CONTRIBUTING.md

@@ -113,7 +113,7 @@ The `main` branch is protected and requires:
 ### Continuous Integration
 
 Every push triggers:
-- Tests on Go 1.21, 1.22, 1.23
+- Tests on Go 1.24 and 1.25 (latest two releases)
 - Linting with golangci-lint
 - Build verification
 - Cross-compilation checks

README.md

@@ -209,7 +209,7 @@ just build-all # Cross-compile for multiple platforms
 
 ### For Building from Source
 
-- Go 1.21+
+- Go 1.24+ (we support and test the latest two Go releases: 1.24 and 1.25)
 - `just` (optional, for using the justfile)
 
 ## How It Works