[M2] Minimal Linux CRT shim (_start) and freestanding CI job

[flyingrobots]

Summary

---

References: #46 (#46)

Note: GITLEDGER_USE_NOSTDLIB remains default-OFF until this issue closes.

[flyingrobots]

✅ Proceed with the M2 CRT shim.

Implement the minimal _start → main path for Linux x86_64 as drafted.
Keep GITLEDGER_USE_NOSTDLIB default OFF in the main matrix, and add a dedicated freestanding-linux CI job to exercise it.
Target “build & exit 0” for now — no fancy I/O or argv/envp handling yet.
Once that passes, we’ll remove the temporary exemption and enforce -nostdlib everywhere.

Why now

• Policy bot requires -nostdlib to really work; the CRT shim is the correct way to validate it.
• Draft in docs/ISSUE-DRAFTS.md documents spec, deliverables, and CI steps.
• Scope is small and self-contained; unblocks the merge and future freestanding builds.

Scope guardrails

• Deliverable: one crt/linux/x86_64/crt0.S + small CMake/Meson wiring.
• No libc emulation, malloc, or printf.
• Smoke test: link and run version_test under -nostdlib and confirm it exits 0.
• CI: Ubuntu-only lane; do not change macOS/Windows lanes yet.

Outcome

• CodeRabbit goes green again.
• Default paths stay stable.
• We get a real, auditable freestanding mode.

Action: implement per the M2 draft and keep the flag OFF by default until the freestanding-linux job is green.