AGENTS.md: document amend!, fixup-only PRs, and direct GfW PRs (#6232)

This closes two gaps in the current `AGENTS.md` that came up while
drafting #6231 and #2104:

1. The `Fixup Commits` section only covered `fixup!`. `amend!` has
different semantics (replaces the target's commit message and combines
diffs to produce any final state), and the "amend! whose body is the
upstream commit and whose diff aligns the squashed result with that
upstream commit" pattern is precisely how the first commit of #6231
anticipates the in-flight nedmalloc removal in `seen` as `e576abb9f8`.
That pattern is undocumented today.

2. There is no top-level "Contributing to Git for Windows" section, only
"Contributing to Upstream Git via GitGitGadget". The cross-fork `gh pr
create` invocation, the conditions under which a PR is naturally a
fixup/amend-only series against existing thicket commits, and the
upstream-aligning `amend!` shape were all things I inferred from context
rather than the guide.

Add subsections that cover those gaps. Nothing in the existing text
changes.

Author: dscho

AGENTS.md

@@ -859,6 +859,53 @@ gets squashed into.
 
 Run affected tests before finalizing.
 
+### `amend!` Commits
+
+A `fixup!` commit keeps the target's commit message and merely combines
+its diff into the target. An `amend!` commit additionally **replaces**
+the target's commit message with its own body. Use `amend!` when the
+fix changes the meaning of the target sufficiently that the original
+subject or body is no longer accurate, or when the goal is to align a
+downstream commit with a specific upstream replacement.
+
+The format is rigid: the first line of an `amend!` commit must be
+exactly `amend! <subject of target>`, followed by a blank line and then
+the **new** commit message that should replace the target's, starting
+with the new subject line:
+
+```
+amend! mingw: use mimalloc
+
+mingw: stop using nedmalloc
+
+The vendored nedmalloc allocator under compat/nedmalloc/ has been
+unmaintained upstream...
+```
+
+After autosquash, the resulting commit has the new subject (`mingw:
+stop using nedmalloc`), the new body, and a diff that is the
+composition of the target's diff and the `amend!`'s diff. Crafting the
+`amend!` diff so that the composition equals a known upstream commit's
+diff is the canonical way to align a downstream branch-thicket commit
+with an in-flight upstream replacement: when the next merging-rebase
+picks up the upstream commit, the byte-identical downstream commit
+collapses into it cleanly.
+
+### PRs Composed Entirely of `fixup!` and `amend!` Commits
+
+Adjusting or removing a feature that lives in the branch thicket is
+often best expressed as a PR that consists *only* of `fixup!` and
+`amend!` commits targeting the existing thicket commits. Each pair
+autosquashes during the next merging-rebase. Pairs whose diffs cancel
+exactly produce empty commits, which the rebase drops with
+`--empty=drop`. The end state is *as if the original commits had been
+edited or removed in place*, while preserving review-friendly atomic
+patches in the PR.
+
+This is the preferred pattern for reverting a multi-commit downstream
+feature. Order the fixups in **reverse** of the originals so each
+revert applies cleanly to the worktree as you build the series.
+
 ### Common Adaptation Patterns
 
 **Struct field moves**: When upstream moves fields between structs, update
@@ -948,6 +995,54 @@ On Windows, `unsigned long` is 32 bits even on 64-bit systems. Use `size_t`
 for sizes that may exceed 4GB. Be careful with format strings: use `PRIuMAX`
 with a cast for `size_t` values.
 
+## Contributing to Git for Windows
+
+The primary contribution path for this fork is a PR against
+`git-for-windows/git`'s `main` branch. The repository is laid out as a
+branch thicket on top of an upstream Git base; see
+[Merging-Rebases](#merging-rebases) and
+[Analyzing Branch Thickets](#analyzing-branch-thickets) for the
+mechanics.
+
+### Opening a PR
+
+Push the topic branch to a personal fork on GitHub, then:
+
+```bash
+gh pr create \
+  --repo git-for-windows/git \
+  --base main \
+  --head <you>:<branch> \
+  --title "<subject>" \
+  --body-file <path/to/body.md>
+```
+
+Unlike upstream contributions, the PR body is rendered as Markdown on
+GitHub, not sent as email. Use the formatting that aids review:
+fenced code blocks, tables, links to workflow runs.
+
+### When the PR Adjusts the Thicket Itself
+
+If the PR's purpose is to edit, remove, or replace existing
+branch-thicket commits, the natural form is a series of `fixup!` or
+`amend!` commits targeting the affected originals. See
+[Fixup Commits](#fixup-commits),
+[`amend!` Commits](#amend-commits), and
+[PRs Composed Entirely of `fixup!` and `amend!` Commits](#prs-composed-entirely-of-fixup-and-amend-commits).
+The merging-rebase that produces the next `main` autosquashes these
+into the thicket; the PR exists for review of the individual
+adjustments.
+
+### When an Upstream Patch Will Replace a Thicket Commit
+
+If an upstream patch is in flight (for instance, on `gitgitgadget/git`
+in `seen` or `next`) that replaces a downstream thicket commit, an
+`amend!` commit whose body is a verbatim copy of the upstream commit
+message and whose diff aligns the autosquashed target with the
+upstream commit's diff is the canonical pattern. The next
+merging-rebase that picks up the upstream commit will recognize the
+two as byte-identical and collapse them.
+
 ## Contributing to Upstream Git via GitGitGadget
 
 ### Overview