chore: add pre-push preflight + setup-hooks

Four-block pre-push gate matching the HellionChat pattern:

- Block A (verify-version-consistency.sh): csproj <Version> vs
  repo.json AssemblyVersion / TestingAssemblyVersion / DownloadLink*
  tag presence. Tolerant of repo.json being absent so v0.1.0 (which
  has no public release manifest yet) does not fail at push time;
  the missing-file path turns into the full cross-check once repo.json
  lands.
- Block B: dotnet build Anvil.sln -p:Platform=x64 -c Release. Platform
  pin is forge-wide (Forgeimizer v0.1.0 lesson: solution build defaults
  drift to AnyCPU otherwise).
- Block C: dotnet csharpier check ./Anvil. Catches the accumulated
  formatter drift that hit HellionChat v1.5.6 (12 files) when only
  build was checked per task.
- Block D: markdownlint-cli2 over the repo's *.md files (excludes node_modules,
  bin, obj, .claude, CLAUDE.md).

Plus setup-hooks.sh as the one-shot installer that points
core.hooksPath at .githooks/ and chmods the scripts.

README.md: MD040 fix for the custom-repo URL fence (added text language tag).

scripts/preflight.sh

@@ -0,0 +1,31 @@
+#!/usr/bin/env bash
+# preflight.sh — pre-push gate for the Anvil plugin repository.
+#
+# Block A — version consistency between Anvil.csproj <Version> and the
+#           public repo.json manifest (when it exists).
+# Block B — dotnet build -p:Platform=x64 on the solution to catch
+#           compile-time API drift.
+# Block C — dotnet csharpier check against Anvil/ for C# format drift
+#           (per feedback_hellion_chat_format_check_per_task: accumulated
+#           formatter drift in HellionChat v1.5.6 hit 12 files at push
+#           time when only build was checked per task).
+# Block D — markdownlint-cli2 against the repo's *.md files.
+
+set -euo pipefail
+ROOT="$(cd "$(dirname "$0")/.." && pwd)"
+cd "$ROOT"
+
+echo "==> preflight: Block A — version consistency"
+./scripts/verify-version-consistency.sh
+
+echo "==> preflight: Block B — plugin compile health"
+dotnet build Anvil.sln -p:Platform=x64 --configuration Release --nologo --verbosity quiet
+
+echo "==> preflight: Block C — csharpier reflow check"
+dotnet csharpier check ./Anvil
+
+echo "==> preflight: Block D — markdownlint"
+# npx --yes avoids a global install; first run caches into ~/.npm/_npx/.
+npx --yes markdownlint-cli2 "**/*.md" "#node_modules" "#bin" "#obj" "#.claude" "#CLAUDE.md"
+
+echo "==> preflight: ALL GREEN"