Remove ineffective arbitrary-want setting

[behinddwalls]

Summary

Why?

Partial-clone lazy fetches require direct object wants, while the configuration claimed they could be disabled even though both upload-pack engines always accepted them.

What?

Remove the inert git.allow_any_sha1_in_want option and update tests and integrity documentation to describe arbitrary object wants as part of partial-clone support.

Test Plan

✅ cargo test -p walgit-server --test e2e partial_clone_tree_zero_and_depth_with_filter -- --exact
✅ cargo fmt --all -- --check
✅ git diff --check

Issue

Closes #77

[0bserver07]

CI is waiting on approval for this stack, so I ran the whole thing locally on top of main. Tests, e2e and the sims all pass. Clippy fails on one lint that comes from #86 (manual_let_else in middleware.rs), so that one would go red in CI.

#85, #87, #89, #91 and #92 look good to me and could each go in on their own if rebased on main. #87 needs that rebase anyway since its check sits in the middle of #86's lines. For this one it might be worth a line in the docs saying read access already lets anyone fetch any stored object by id, unreachable ones included, so nobody reads the removal as dropping a protection.

[behinddwalls]

Added the clarification in b0b18b1 that a principal with repository read access could already fetch any stored object by ID, including objects unreachable from advertised refs. Removing git.allow_any_sha1_in_want therefore does not widen authorization; it only removes a non-functional configuration switch. The targeted partial-clone test passes.

[addressed by agent]

[behinddwalls]

Reworked the reviewed branches as independent changes on main: #87 is now 77ade75, #89 is 129fcc2, #91 is a200384, and #92 is ef1120d; #85 was already based on main at b0b18b1. I also removed the stale stack sections from all five PR descriptions. Each rewritten branch now contains only its own change, and its targeted test plus formatting/diff checks pass. #86, #88, and #90 were left untouched.

[addressed by agent]