ci(installer): add pr-checks, release, and project-tree sync workflows

Bring this repo's CI up to parity with the other Runic Gateway repos. All
three are retargeted from RunicGateway/link, which is the closest analog
(same Rust toolchain, same release engine, same runner).

pr-checks.yml
  Gates PRs into main on cargo fmt --check, clippy -D warnings, and
  cargo test --locked, in that order, one job — mirroring release.yml's
  gates so a green PR implies a green release.

release.yml
  The conventional-commit release engine from link/, with the Rust
  adapter retargeted: crate at the repo root, binary
  runicgateway-installer, cross-compiled for x86_64 Linux and Windows.
  Artifact names follow PLAN.md §3. The generated changelog now carries
  the checksum-verification block, because releases are deliberately
  unsigned and SHA256SUMS is the trust anchor (PLAN.md §3) — that makes
  the verify instructions part of the release, not a doc someone has to
  find.

sync-project-tree.yml (+ .gitea/scripts/gen_tree.py)
  Regenerates docs/installer/PROJECT_TREE.md on every push to main and
  opens or force-updates a PR against the docs repo. Verbatim from link/
  apart from the repo/path/label env block.

Crate guard
  This repo has no Cargo project yet — Phase 1 creates it. Landing the
  workflows unguarded would red-X every governance and docs PR until
  then, and holding them back leaves the repo ungated exactly while its
  conventions are being set. So both Rust workflows check for a root
  Cargo.toml first: pr-checks skips its gates with a notice, and
  release.yml's plan step sets RELEASE=false and exits. Both arm
  themselves the moment Cargo.toml lands, with no edit here.

Verified before pushing: all three files parse as YAML, every run block
passes bash -n, and the release plan step was simulated against a throwaway
git repo both without a crate (release=false, exit 0) and with one
(first-release path -> v0.1.0 with the changelog rendered).

Not included: the bundle-manifest workflow (PLAN.md §7) and the
release-dispatch hook, which are Phase 0 item 3 and depend on
servuo-plugins having a release workflow first.

Note for setup: release.yml and sync-project-tree.yml need REGISTRY_USER
and REGISTRY_TOKEN (write:repository, plus read/write on RunicGateway/docs)
configured for this repo.

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

.gitea/scripts/gen_tree.py

@@ -0,0 +1,54 @@
+#!/usr/bin/env python3
+"""Render an ASCII tree of tracked files, read from stdin (one path per line).
+
+Used by the `sync-project-tree` workflow to regenerate this repo's PROJECT_TREE.md
+snapshot in the RunicGateway/docs repo. Feed it `git ls-files`:
+
+    git ls-files | python3 .gitea/scripts/gen_tree.py <root-label>
+
+Deterministic ordering: directories before files, each group sorted
+case-insensitively with the raw name as a tiebreak. Output uses the classic
+`tree(1)` box-drawing style so the result is stable across runs and platforms.
+"""
+import sys
+
+
+def build(paths):
+    root = {}
+    for p in paths:
+        p = p.strip().replace("\\", "/")
+        if not p:
+            continue
+        node = root
+        for part in p.split("/"):
+            node = node.setdefault(part, {})
+    return root
+
+
+def render(node, prefix, lines):
+    entries = list(node.items())
+    # directories (non-empty children dict) before files, then case-insensitive name
+    entries.sort(key=lambda kv: (0 if kv[1] else 1, kv[0].lower(), kv[0]))
+    for i, (name, child) in enumerate(entries):
+        last = i == len(entries) - 1
+        branch = "└── " if last else "├── "
+        suffix = "/" if child else ""
+        lines.append(f"{prefix}{branch}{name}{suffix}")
+        if child:
+            render(child, prefix + ("    " if last else "│   "), lines)
+
+
+def main():
+    try:
+        sys.stdout.reconfigure(encoding="utf-8", newline="\n")
+    except AttributeError:
+        pass
+    root_label = sys.argv[1] if len(sys.argv) > 1 else "."
+    tree = build(sys.stdin.read().splitlines())
+    lines = [f"{root_label}/"]
+    render(tree, "", lines)
+    sys.stdout.write("\n".join(lines) + "\n")
+
+
+if __name__ == "__main__":
+    main()