Improve source-of-truth detection for adoption analysis

[Zach677]

Context

The analyzer should avoid adding a parallel workflow when a repo already has a source of truth for project generation, dependency management, or validation.

Scope

Improve detection for common workflow surfaces:

• XcodeGen (project.yml, generated project patterns)
• Fastlane
• Bazel / Buck
• Just
• GitHub Actions workflows
• custom scripts/ validation entrypoints
• existing Makefile / mise interactions

Acceptance

• preserve-existing-workflow does not recommend adding conflicting command surfaces.
• JSON output explains the detected source_of_truth or existing command surfaces clearly enough for another Agent to act safely.
• Regression tests cover at least Fastlane, XcodeGen, and one custom script workflow.

[linear-code]

SPA-56

[Zach677]

Scoping this down: XcodeGen, Fastlane, and custom scripts/ entrypoints are the surfaces this template's audience actually hits, so they stay in scope. Bazel and Buck detection is deferred until a real repository that uses them shows up.

Related note for whoever picks this up: SKILL.md's Scenario Guidance section duplicates some of the analyzer's advice prose. When adding new detection surfaces, single-source that guidance instead of growing both copies.