feat(apex): expose runtime adapter through Codex CLI build

[kratosboom]

DOGFOOD REACHABILITY CLOSURE v0.1

PHASE A:
INTERACTIVE CLI RUNTIME REACHABILITY PROVEN

Normal codex enters codex_tui::run_main, which starts an embedded app-server by default. codex exec also starts an in-process app-server. The app-server feature path reaches ClientRequest::FsReadFile and runs the Apex Runtime Adapter before FsRequestProcessor::read_file.

PHASE B:
FEATURE PROPAGATION IMPLEMENTED

Added the non-default codex-cli feature:
apex-runtime-adapter = ["codex-app-server/apex-runtime-adapter"]

RUNTIME ADAPTER BUILD EXPOSURE ONLY
NO SAFETY KERNEL CHANGE
NO RUNTIME IMPLEMENTATION CHANGE
NORMAL CODEX BUILD REMAINS UNCHANGED
NO PRODUCTION DEPLOYMENT
NO MERGE

VALIDATION:

• normal CLI build PASS
• adapter-enabled CLI build PASS
• normal/feature/all-features cargo check PASS
• safety kernel 76/76 PASS
• runtime adapter 10/10 PASS
• focused app-server 3/3 PASS
• filesystem regression 14/14 PASS
• fmt check PASS
• diff check PASS

DOGFOOD:
Separate adapter-enabled binary: C:\ApexCode\bin\apexcode.exe
Normal installed Codex was not modified.
User mode: normal interactive apexcode; explicit apexcode app-server is also available.
This is a reviewed dogfood candidate, not a production-ready claim.

No merge.