Multiple files hardcode execFileSync/execSync maxBuffer instead of registering in DEFAULTS

[carlos-alm]

While fixing the DEFAULTS-registration gap for resolveSecrets' execFileSync options in src/infrastructure/config.ts (gauntlet rule 7, Titan phase 7), noticed the same magic-number pattern repeated elsewhere in the codebase — each hardcodes a maxBuffer value inline for a subprocess call instead of sourcing it from DEFAULTS:

• src/features/cochange.ts:57 — maxBuffer: 50 * 1024 * 1024
• src/features/check.ts:345 — maxBuffer: 10 * 1024 * 1024
• src/domain/analysis/diff-impact.ts:52 — maxBuffer: 10 * 1024 * 1024
• src/domain/graph/builder/stages/native-orchestrator.ts:1467 — maxBuffer: 100 * 1024 * 1024

Per CLAUDE.md's convention ("never introduce new hardcoded magic numbers in individual modules... always add them to DEFAULTS"), these should each get a named DEFAULTS entry (grouped under whichever config section is most appropriate per file, e.g. DEFAULTS.check, DEFAULTS.coChange) instead of an inline literal, mirroring the fix applied to config.ts's resolveSecrets.

Out of scope for the current phase (targets only src/infrastructure/config.ts) — filed separately per scope-discipline policy.