fix: Update minimum Go version requirement to 1.23

Fixes #34

Problem: Users with Go 1.18 were unable to install beads because go.mod
specifies 'go 1.23.0', which older Go toolchains cannot parse. The error
"invalid go version '1.23.0': must match format 1.23" occurs because
Go versions before 1.21 don't support the three-part version format.

Root cause: Our dependencies (especially modernc.org/sqlite) require Go 1.23,
which forces go.mod to use the 1.23.0 format. This is correct for our actual
requirements, but was incorrectly documented as requiring only Go 1.21.

Solution:
1. Updated install.sh to check for Go 1.23+ and show clear error messages
   if an older version is detected
2. Updated README.md to correctly state "requires Go 1.23+" instead of 1.21+
3. go.mod already correctly specifies go 1.23.0 (no changes needed there)

The version check in install.sh now:
- Parses the Go version from 'go version' output
- Compares major.minor version numbers
- Fails fast with helpful upgrade instructions if Go < 1.23

This prevents confusing build errors and guides users to upgrade Go before
attempting installation.

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

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

README.md

@@ -74,7 +74,7 @@ The installer will:
 ### Manual Install
 
 ```bash
-# Using go install (requires Go 1.21+)
+# Using go install (requires Go 1.23+)
 go install github.com/steveyegge/beads/cmd/bd@latest
 
 # Or build from source