fix(vcs): avoid unbounded diff memory usage

[nexxeln]

summary

• add capped native git patch helpers for tracked, batched, and untracked diffs
• switch /vcs/diff to batched native git patches with per-file and total response safeguards
• keep the existing VCS diff response shape while avoiding JS full-file patch generation

Addresses the /vcs/diff memory pressure called out in #22683 and #24049

testing

• bun test test/git/git.test.ts
• bun test test/project/vcs.test.ts
• bun typecheck
• push hook: bun turbo typecheck