Improve git protocol parsing and error handling

[pjbgf]

Summary

• Surface remote ERR packets during v2 fetch and validate the acknowledgment flow (delimiter, packfile section, premature flush) so transport failures aren't silently swallowed.
• Tighten pkt-line framing: accept empty data packets (length 0004) per spec, and reject packets exceeding the 65520-byte limit.
• Validate smart HTTP responses by normalizing endpoint paths and checking the info/refs advertisement Content-Type before parsing.
• Skip unborn entries in ls-refs output rather than treating them as refs.
• Rename the README "limits" section for clarity.

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Note

Medium Risk
Medium risk because it changes low-level Git protocol parsing/validation (pkt-line framing, v2 fetch negotiation, smart HTTP discovery), which can affect interoperability with servers and error handling paths.

Overview
Improves robustness of Git protocol v2 handling by surfacing remote ERR packets and validating the v2 fetch acknowledgment flow (including enforcing ready → packfile sequencing and rejecting premature flush/invalid sections).

Tightens pkt-line decoding to accept empty data packets (0004) while rejecting lengths beyond the pkt-line max, skips unborn entries in v2 ls-refs, and strengthens smart-HTTP discovery by normalizing trailing slashes in endpoint paths and requiring the expected info/refs advertisement Content-Type before parsing. Documentation renames README “Limits” to “Limitations”.

^{Reviewed by Cursor Bugbot for commit 5890a3d. Configure here.}

[Soph]

Thanks a lot!