[Repo Assist] feat: add %o octal format and # flag to l_vsnprintf

[github-actions]

🤖 This is an automated pull request from Repo Assist.

Summary

Add two missing format features to l_vsnprintf:

• %o / %lo / %llo: octal integer formatting
• # flag for %x/%X (adds 0x/0X prefix for nonzero values)
• # flag for %o (adds 0 prefix for nonzero values)

These are required by POSIX/C99 printf conformance and are commonly used for file permission display (%04o) and hex dumps.

Changes

• l_os.h: adds flag_hash to the flag-parsing loop, adds %o to the numeric dispatcher, extends the prefix logic for #x, #X, #o
• tests/test_strings.c: 5 new test cases covering basic octal, zero, permissions, zero-pad, and # flag

Test Status

✅ Linux/gcc: build PASS, 1515 assertions PASS
✅ Linux/clang: build PASS, 1513 assertions PASS
✅ Verify: 52 binaries, max 117KB, static PASS

Generated by 🌈 Repo Assist at {run-started}. Learn more.

To install this agentic workflow, run

gh aw add githubnext/agentics/workflows/repo-assist.md@1f672aef974f4246124860fc532f82fe8a93a57e