docs: add SECURITY.md and GitHub issue templates

Add a security policy directing private vulnerability reports to
security@astrolabecloud.com instead of public issues, and update the
README's Security section to point at it.

Add structured issue forms under .github/ISSUE_TEMPLATE/ covering bugs,
feature requests, questions, and documentation, plus a config.yml that
disables blank issues and routes security reports and open-ended
questions to the appropriate channels. The bug template captures
fields most commonly missing from past reports (server/Nextcloud/app
versions, deployment mode, transport, MCP client).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

SECURITY.md

@@ -0,0 +1,18 @@
+# Security Policy
+
+## Reporting a Vulnerability
+
+**Please do not report security vulnerabilities through public GitHub issues.**
+
+If you discover a security vulnerability in this project, report it privately by emailing:
+
+**security@astrolabecloud.com**
+
+Please include as much of the following information as possible to help us triage your report:
+
+- A description of the vulnerability and its potential impact
+- Steps to reproduce the issue (proof-of-concept code, if applicable)
+- The version(s) of the project affected
+- Any known mitigations or workarounds
+
+We will acknowledge receipt of your report and work with you on coordinated disclosure.