Keep Git transfers and readers compatible with complete stored history

[nishu-builder]

This PR has been split into three smaller PRs below the publication design:

• Prevent :@@= loads from (falsely) indicating to the server they have missing commit ancestry #260 — Keep snapshot fetches from making client repositories shallow.
• Keep server object reads working as Git packs accumulate #258 — Keep server object reads working as Git packs accumulate.
• Allow fetching imported commits before any branches exist #259 — Allow fetching imported commits before any branches exist.

Each replacement explains the failure it fixes and carries its relevant regression tests. The publication design is #248, above these fixes. The combined code is unchanged.

GitHub automatically marked this PR merged into the design branch during reordering because that branch contained its rewritten head. Nothing was merged into main. The fixes remain open for review in the three PRs above.

[nishu-builder]

Layer 2/8. The HTTP_GIT_PROTOCOL pass-through, the alternates-after-staging reorder in validate_commit_with_git, and the consumer-fetch and 40-pack assertions in the fixture all look right and are the kind of thing that only shows up under load.

Two inline comments, both about cost rather than correctness: dropping --depth 1 makes every :@@= resolution (including the cloud install's --llm-step locator) download the target repo's full history, and Slots::Given(32767) is now an absolute ceiling on pack count in a store that nothing ever repacks.

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