feat(extraction): make MAX_FILE_SIZE configurable via CODEGRAPH_MAX_FILE_SIZE env var

[fengshao1227]

Summary

Projects with legitimate large source files (1–5 MB hand-written code, not minified bundles or vendored blobs) are silently skipped during indexing due to the hardcoded 1 MB file size limit. This leaves gaps in the function graph.

This PR makes the limit configurable via the CODEGRAPH_MAX_FILE_SIZE environment variable (value in bytes), following the existing pattern of CODEGRAPH_* env vars for runtime configuration:

CODEGRAPH_MAX_FILE_SIZE=5242880 codegraph index   # 5 MB limit

The 1 MB default is unchanged — zero behavior difference for users who don't set the variable.

• Invalid values (non-numeric, zero, negative) silently fall back to the 1 MB default
• The size_exceeded warning message already includes the effective limit, so users can see the configured value
• The exported getMaxFileSize() function enables testing and future use by codegraph status

Closes #1016

Test plan

• 3 new tests in extraction.test.ts:
  • Default 1 MB when env var is not set
  • Custom value when env var is a valid positive integer
  • Fallback to default for invalid values (non-numeric, negative, zero)
• All existing extraction tests pass (zero regressions)