chore(runners): deduplicate parseSemver/compareSemver — 3 copies in codebase

[OneStepAt4time]

Problem

parseSemver and compareSemver are implemented independently in 3 locations:

1. src/validation.ts (canonical, 973-line file)
2. src/runners/cc-agents-discovery.ts (local copy, 218-line file)
3. src/commands/update.ts (local parseSemver, different isNewer wrapper)

Impact

• Bug fix in one copy may not propagate to others
• Different implementations may diverge in edge-case handling
• Pre-existing tech debt that feat: multi-agent support — strategic gap exposed by OpenACP (28+ agents) and cc-connect (10+ agents) #3180 implementation would inherit

Fix

• Extract to src/utils/semver.ts
• Re-export from validation.ts for backward compat
• All consumers import from src/utils/semver.ts

Scope

Low risk, no behavior change. Tech-debt cleanup.

Discovered during

Code quality sweep on src/runners/ (pre-#3180 audit).