[STORY] v4-S26: Implement v3 config compatibility layer

[raifdmueller]

User Story

As a documentation author upgrading from v3, I want my existing docToolchainConfig.groovy to work without changes.

Acceptance Criteria

Scenario: v3 config with Gradle-specific settings
  Given a v3 docToolchainConfig.groovy with Gradle-era settings
  When I run ./dtcw generateHTML on v4
  Then Gradle-specific settings are ignored silently
  And generation succeeds with the same output as v3

Scenario: v3 config with all features
  Given a v3 config using inputFiles, imageDirs, confluence, jira, microsite sections
  When I run v4 tasks
  Then all configured features work correctly

Priority: Must

Epic: v4-E5 (#7)

Blocked by: v4-S3

[raifdmueller]

Analysis

Config Format Is Stable

docToolchainConfig.groovy uses Groovy ConfigSlurper — framework-agnostic. No changes needed.

What dtcw Must Handle

• gradle.properties settings that users may have customized (e.g., -Xmx2048m) → move to dtcw defaults or env vars
• mainConfigFile property → dtcw already has DTC_CONFIG_FILE env var
• Custom Gradle properties passed via -P flags → map to system properties (-D)

Estimated: ~30 LOC in dtcw (property mapping)

🤖 Generated with Claude Code