codeSplitter merge 2: a build-checked AI fallback, and refuse conflict markers #934

Design: apps/codesplitter/docs/merge-solutions.md, §6 and §7.

• conflict-agent.ts runs only on what L1–L3 leave, which was 3 of 29 hunks in the measurement. It gets the hunk, both sides' commit messages and the already-settled merge of the rest of the file.
• Its answer is offered only if the file parses, the build passes, and the touched files' tests pass in a throwaway clone (the doctor's tester already does this). If not, the hunk goes to a person with the AI's attempt and the failure attached.
• merge() refuses a side that contains conflict-marker lines, and a keyed file (JSON, YAML, TOML) that doesn't parse after the merge. codeSplitter dependency doctor: 🪏 dig a future release to watch, try, and take (#863) #884's squash merge committed <<<<<<< HEAD into src/features.json (fixed in the PR that adds the doc). codeSplitter's merge would have carried those markers through as a "clean" merge.
• Optional: a CI check that fails on conflict-marker lines anywhere in apps/.