security: add vulnerability disclosure policy (security.txt + SECURITY.md)

A security product whose agent runs as root had no way to receive a private
report. Adds an RFC 9116 security.txt (served at /.well-known/security.txt)
and a SECURITY.md that GitHub surfaces as the repo's Security tab: a contact,
a private-first disclosure ask, the in-scope surfaces, and what we commit to
(signed-before-parse, degrade-safe, no telemetry). Closes the last open
trust gap from the review.

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

SECURITY.md

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+# Security policy
+
+Hound is a security product, and its agent runs as a root daemon that can
+block execution. That earns a low bar for reporting problems and a high bar
+for how we handle them.
+
+## Reporting a vulnerability
+
+Email **security@houndav.com** with enough detail to reproduce. Please report
+privately and give us a chance to ship a fix before public disclosure. You
+will get an acknowledgement, and we will keep you updated as we work it.
+
+In scope:
+
+- The agent (`houndd`, `hound`, `hound-gui`, `hound-mcp`) — memory safety,
+  privilege escalation, the execution gate, the fanotify path, the MCP server.
+- The definitions pipeline — signing, verification, the update client, pack
+  and licence parsing.
+- The download and definitions hosts, and this project's release artifacts.
+
+Especially interesting: anything that lets an unsigned or tampered definition
+pack, rules pack, or release manifest be accepted; any way to make the agent
+quarantine or block something it should not; any path traversal or RCE reachable
+through a pack, a licence token, or the update flow.
+
+## What we commit to
+
+- Definitions, rules packs, and release manifests are Ed25519-signed and
+  verified **before** parsing, against a key whose public half is compiled into
+  the agent. Verification failing degrades to a safe state, never to a broken
+  one.
+- No telemetry: the agent's only network call is fetching signed packs, and
+  that can be pointed at your own mirror.
+- We will credit reporters who want it, and we will not pursue good-faith
+  research conducted under this policy.
+
+## Verifying a report against the source
+
+The agent is Apache-2.0 and public at
+<https://github.com/HoundAV/hound-antivirus>. You are being asked to trust a
+root daemon; you should be able to read exactly what it does.

site/.well-known/security.txt

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+# Hound Antivirus — vulnerability disclosure policy
+# https://houndav.com/.well-known/security.txt (RFC 9116)
+
+Contact: mailto:security@houndav.com
+Expires: 2027-08-21T00:00:00.000Z
+Preferred-Languages: en
+Canonical: https://houndav.com/.well-known/security.txt
+Policy: https://github.com/HoundAV/hound-antivirus/blob/main/SECURITY.md
+
+# Hound is a security product and its agent runs as root. If you have found a
+# vulnerability in the agent, the definitions pipeline, or this site, please
+# report it privately to the address above before disclosing it publicly, and
+# we will work the fix with you. The source is Apache-2.0 and public:
+# https://github.com/HoundAV/hound-antivirus