Add ctxguard: hook-enforced context sanitization

Keeps credentials, company names, personal names and PII out of the model's
context. Enforcement lives in Claude Code hooks rather than in instructions to
the agent: a skill alone cannot protect anything, because by the time the agent
reads a rule the surrounding context has already been sent.

Credentials are removed irreversibly and marked. Entities from a user-supplied
dictionary become stable aliases, rewritten back to real values on their way to
disk and to the shell, so code and commands referring to them still work.

Published from a clean tree; development history is not included.

plugins/ctxguard/commands/ctx-sanitize.md

@@ -0,0 +1,19 @@
+---
+description: Turn a block of text (ticket, log, email) into an aliased version safe to paste
+---
+
+The user has text containing sensitive data and needs an aliased version they can
+send. A hook cannot rewrite a prompt, only block it, so this is the supported route.
+
+Ask the user to paste the text into a file (or use text they already have on disk),
+then run:
+
+```
+python3 "${CLAUDE_PLUGIN_ROOT}/scripts/ctxguard.py" sanitize < <file>
+```
+
+The aliased text goes to stdout; a per-rule summary goes to stderr.
+
+Note: only values already in the dictionary become aliases. Credentials and PII are
+detected by pattern regardless. If a company or person came through untouched, it is
+not registered yet — offer `/ctx-entity`.