[0.1.5] Add docscribe capability detection: parse version, skip server on < 1.5.1

[unurgunite]

Description

Adds capability detection for docscribe gem version. When performGemCheck() runs bundle exec docscribe --version, the output is now parsed into a structured DocscribeCapabilities (version + serverMode flag). ensureRunning() checks capabilities before attempting server startup — versions before 1.5.1 skip the server and fall back directly to CLI, avoiding unnecessary startup attempts and errors.

Changes

• DocscribeCapabilities — new data class with version: String and serverMode: Boolean
• performGemCheck() — captures --version stdout, calls parseVersion() to build capabilities
• parseVersion() — companion method that parses semver, enables serverMode for ≥ 1.5.1
• ensureRunning() — returns null (CLI fallback) when capabilities indicate no server mode
• Tests — 9 new tests: version parsing for 1.5.0 (no server), 1.5.1 (server), 1.6.0, 2.0.0, 1.4.9, trailing newline, garbage input, null default, and integration (no crash when server mode disabled)

Checklist

• Tests pass (./gradlew test)
• Formatting clean (./gradlew spotlessCheck)
• Static analysis clean (./gradlew detekt)
• Plugin verified (./gradlew verifyPlugin)
• CHANGELOG updated