misc: add install-git-hooks.go and git hook for Change-Id tracking

Add misc/install-git-hooks.go and misc/git_hook/ to the OSS repo,
adapted from the corp repo. The primary motivation is Change-Id
generation in commit messages, which provides a persistent identifier
for a change across cherry-picks between branches.

The installer uses "git rev-parse --git-common-dir" instead of go-git
to find the hooks directory, avoiding a new direct dependency while
still supporting worktrees.

Hooks included:
- commit-msg: adds Change-Id trailer
- pre-commit: blocks NOCOMMIT / DO NOT SUBMIT markers
- pre-push: blocks local-directory replace directives in go.mod
- post-checkout: warns when the hook binary is outdated

Also update docs/commit-messages.md to reflect that Change-Id is no
longer optional in the OSS repo.

Updates tailscale/corp#39860

Change-Id: I09066b889118840c0ec6995cc03a9cf464740ffa
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

go.mod

@@ -328,7 +328,7 @@ require (
 	github.com/emirpasic/gods v1.18.1 // indirect
 	github.com/ettle/strcase v0.2.0 // indirect
 	github.com/evanphx/json-patch/v5 v5.9.0 // indirect
-	github.com/fatih/color v1.18.0 // indirect
+	github.com/fatih/color v1.18.0
 	github.com/fatih/structtag v1.2.0 // indirect
 	github.com/firefart/nonamedreturns v1.0.4 // indirect
 	github.com/fsnotify/fsnotify v1.9.0
@@ -448,7 +448,7 @@ require (
 	github.com/sivchari/tenv v1.7.1 // indirect
 	github.com/skeema/knownhosts v1.3.1 // indirect
 	github.com/sonatard/noctx v0.0.2 // indirect
-	github.com/sourcegraph/go-diff v0.7.0 // indirect
+	github.com/sourcegraph/go-diff v0.7.0
 	github.com/spf13/afero v1.11.0 // indirect
 	github.com/spf13/cast v1.7.0 // indirect
 	github.com/spf13/cobra v1.10.2 // indirect