hooks: branch guard, 300-line cap, task ref, auto PR (V-445)

Four git hooks, tracked in .githooks and wired with core.hooksPath so a
fresh clone gets them with one config line.

pre-commit refuses master and refuses more than 300 changed lines in
non-markdown files. Markdown is exempt because docs land as one batch.
commit-msg requires (V-<id>), not (#<id>), because Gitea autolinks # to
its own issues and that is the wrong tracker. post-checkout records the
parent branch, since git does not track where a branch was cut from and
the PR needs the base. pre-push opens the PR with tea and never blocks.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

scripts/task-branch.sh

@@ -0,0 +1,29 @@
+#!/bin/sh
+# Cut a task branch and record the branch it came from, so pre-push can open
+# the PR against the right base.
+#
+#   scripts/task-branch.sh 359 narrow-single-token
+#   -> task/359-narrow-single-token, parent recorded as the current branch
+
+set -eu
+
+if [ $# -lt 2 ]; then
+	echo "usage: $0 <vikunja-id> <slug>" >&2
+	exit 2
+fi
+
+id=$1
+shift
+slug=$(printf '%s' "$*" | tr '[:upper:] ' '[:lower:]-' | tr -cd 'a-z0-9-')
+
+parent=$(git symbolic-ref --short HEAD)
+branch="task/$id-$slug"
+
+git checkout -b "$branch"
+
+dir="$(git rev-parse --git-dir)/maven-parent"
+mkdir -p "$dir"
+printf '%s\n' "$parent" > "$dir/$(printf '%s' "$branch" | tr '/' '_')"
+
+echo "$branch, cut from $parent"
+echo "Commits on it must end with (V-$id)."