chore(installer): bootstrap repo with governance docs and templates

The installer repo was created empty. Seed it with the same governance set
every other Runic Gateway repo carries, so it starts on the same footing
before any Rust code lands (see docs/installer/PLAN.md for the design of
record — this repo is still in the planning phase).

Copied verbatim, byte-identical to the other repos:

  LICENSE.md (GPL-3.0-or-later), CODE_OF_CONDUCT.md, CONTRIBUTORS.md,
  .gitea/PULL_REQUEST_TEMPLATE.md, .gitea/ISSUE_TEMPLATE/{bug_report,
  feature_request}.md

Repo-specific:

  README.md          what the installer is, what it deliberately is not
                     (no curl|bash, never writes a ServUO launcher), the
                     planned commands, and the constraints a reader needs
                     up front: unsigned releases, bundle-manifest
                     composition, opt-in patch tier, and the fact that a
                     successful copy is not a working bridge.
  CONTRIBUTING.md    adapted from link/ (same Rust toolchain and checks),
                     plus a planning-status note pointing changes of scope
                     at the plan in docs/, and the two shard-testing traps.
  SECURITY.md        adds the installer to the component scope table and a
                     short subsection on its distinct trust model: unsigned
                     releases anchored on SHA256SUMS, mandatory
                     verification of downloaded artifacts, and the
                     never-contacts-the-website token handoff. This is the
                     only file that now differs from the other repos' copies.
  .gitignore         Rust build output plus local deployment state
                     (install.json, sidecar.toml, *.db) that must never be
                     committed from a test run.
  .gitea/ISSUE_TEMPLATE/config.yaml   same as elsewhere, repo-local URL.

No CI workflows yet — there is no crate for pr-checks to build, and the
release/bundle workflows are Phase 0 work that depends on servuo-plugins
gaining a release workflow first.

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

CONTRIBUTORS.md

@@ -0,0 +1,31 @@
+# Contributors
+
+Runic Gateway is built and maintained by the people and tools listed here.
+Thank you to everyone who has contributed.
+
+## Maintainers
+
+- **whitlocktech** <whitlocktech@gmail.com> — project lead and maintainer
+
+## Contributors
+
+<!--
+  Add yourself here when your contribution is merged — alphabetical by name or
+  handle. One line each:
+
+  - **Name or handle** (optional link) — what you contributed
+-->
+
+- _Your name could be here — see [CONTRIBUTING.md](CONTRIBUTING.md)._
+
+## AI-assisted development
+
+Parts of Runic Gateway were developed with the assistance of AI coding tools,
+including **Claude** (Anthropic) via Claude Code. AI-assisted commits are
+attributed in their commit trailers (e.g. `Co-Authored-By: Claude ...`).
+
+In keeping with this project's transparency policy, **all contributors must
+disclose their use of AI tools** on any contribution — see the
+"AI-assisted contributions" section of [CONTRIBUTING.md](CONTRIBUTING.md).
+Disclosed AI assistance is welcome; undisclosed AI-generated contributions are
+not.