fix(runway): ISS-011 fail fast without Git configuration

[sbalabanov]

Summary

Intent:

• Prevent an explicit Git merger selection from silently starting with the noop merger.
• Keep automatic noop fallback and explicit noop/fake overrides unchanged.

Changes:

• Validate startup configuration when MERGER=git and require at least one usable Git target.
• Cover environment and file-based Git configuration, Git-less files, noop, fake, unset, and invalid selections.
• Document the explicit Git startup contract.

Reproduction:

• Deploy Runway with MERGER=git while omitting both MERGE_CONFIG_PATH and MERGE_CHECKOUT_PATH.
• Previously the service started with the noop merger and could publish synthetic successful merge results without changing Git.
• Startup now fails with: MERGER="git" requires usable Git configuration: set MERGE_CHECKOUT_PATH or set MERGE_CONFIG_PATH to a config containing at least one git merger.

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_{Generated by the 🪄 pr-create skill in devexp-agent-marketplace}

Test Plan

AI Verification

Validated at 492963f on Sep 4 22:15 UTC · 4 files analyzed · 1s

Validator	Status	Issues
java-lint	not_applicable	0
java-coverage	not_applicable	0
ios-lint	not_applicable	0
fix-disclosure	not_applicable	0
merge-conflict	not_applicable	0
visual-web	not_applicable	0
go-thrift-lint	not_applicable	0
web-coverage	not_applicable	0
web-unit	not_applicable	0
diff-template	not_applicable	0
go-gazelle	not_applicable	0
arc-unit	not_applicable	0
arc-lint	not_applicable	0
android-coverage	not_applicable	0
web-typecheck	not_applicable	0
web-repocheck	not_applicable	0
go-lint	not_applicable	0
go-coverage	not_applicable	0
visual-ios	not_applicable	0
android-lint	not_applicable	0
ios-test	not_applicable	0
uber-one	not_applicable	0
web-lint	not_applicable	0
visual-android	not_applicable	0
go-proto-lint	not_applicable	0
custom	not_applicable	0
ureview	completed	0

0 issues detected

_{Skipped validators: claude · EngWiki}

Issues

T3-ISS-011

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