[Feature] Replace LibGit2Sharp with a managed Git implementation (managed reads + git CLI writes)

Description

Replace LibGit2Sharp with a hybrid Git backend: a vendored fully managed reader for read/history operations plus the git CLI for writes and network operations. The end state ships no native Git binaries.

Supersedes the exploration in arturcic/GitVersion#236. The full technical design remains in docs/design/managed-git-migration.md and must be aligned with the release plan below.

Status

Phases A, B-pre, and B are complete and merged through #5084. The managed backend is implemented and covered by backend parity tests.

The release lifecycle is now governed by #5135:

The former v7.1 default-flip issue #5039 is superseded by #5135.

Phase tracking

Shipped architecture

• src/GitVersion.Git.Managed contains the managed reader and Git CLI executor and implements the Core Git abstractions directly.
• Read-only version calculation on a prepared checkout does not require git on PATH.
• Repository normalization, dynamic repositories, fetch, clone, checkout, and other network/write operations use the Git CLI.
• Backend selection is centralized during the v7.0 compatibility window.
• #5040 removes libgit2 production code and dual-backend comparison infrastructure in v7.1 while retaining only the v7 selector compatibility validator required by #5135.

Validation requirements

• Full integration suites and order-sensitive parity tests must remain green for both backends until libgit2 removal.
• Managed tests cover loose/packed objects, delta resolution, refs, worktrees, shallow repositories, revwalk ordering, merge-base, tree diff, index/status, and repository preparation.
• Artifact tests verify that v7.1 packages contain no LibGit2Sharp assemblies or runtimes/**/native/* entries.
• Migration and breaking-change documentation must consistently reflect the v7.0 managed default, v7.1 removal, and v8 selector cleanup.

Related follow-ups

• #2921 tracks lightweight dynamic repositories and remote-ref discovery separately.
• #5135 aligns parser, configuration, and Git-backend selectors across v7.
• #5136 removes the selector variables in v8.

Read docs/design/managed-git-migration.md and the release plan in #5135, then inspect src/GitVersion.Git.Managed and the phase issues listed here. The work is complete for the managed backend; remaining validation follows #5040, including full integration and parity suites, artifact checks, and consistent v7/v8 documentation.