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

[gittools-bot]

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#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:

Release	Runtime behavior
v7.0	Managed backend is the default; GITVERSION_GIT_BACKEND=libgit2 is the temporary fallback.
v7.1	LibGit2Sharp and native binaries are removed by #5040; libgit2 selection produces an actionable removal error.
v7.x	Explicit GITVERSION_GIT_BACKEND=managed remains accepted as a no-op for automation compatibility.
v8	#5136 removes the selector variable and its compatibility validation.

The former v7.1 default-flip issue #5039 is superseded by #5135.

Phase tracking

Phase	Issue	State
A — Git CLI mutator	#5032	Complete
B-pre — interface cleanup	folded into #5032	Complete
B.1 — object store	#5033	Complete
B.2 — refs/discovery	#5034	Complete
B.3 — revwalk/merge-base	#5035	Complete
B.4 — diff/status	#5036	Complete
B.5 — adapter and parity	#5037 / #5084	Complete
Former C — v7.1 default flip	#5039	Superseded by #5135
E — remove LibGit2Sharp	#5040	Planned for v7.1
F — commit-graph acceleration	#5041	Optional

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.
• v7.1: remove LibGit2Sharp and native binaries #5040 removes libgit2 production code and dual-backend comparison infrastructure in v7.1 while retaining only the v7 selector compatibility validator required by Align v7 feature selectors and legacy lifecycle #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

• GitVersion unable to work with remote reference and needs local copy of branches #2921 tracks lightweight dynamic repositories and remote-ref discovery separately.
• Align v7 feature selectors and legacy lifecycle #5135 aligns parser, configuration, and Git-backend selectors across v7.
• Remove feature-selector environment variables #5136 removes the selector variables in v8.