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-audit.md

@@ -0,0 +1,16 @@
+---
+description: Show what ctxguard has been blocking and substituting
+---
+
+```
+python3 "${CLAUDE_PLUGIN_ROOT}/scripts/ctxguard.py" audit -n 40
+python3 "${CLAUDE_PLUGIN_ROOT}/scripts/ctxguard.py" status
+```
+
+The audit log records rule ids, tool names and aliases — never plaintext, so it is
+safe to read inside a session.
+
+Read it for false positives: a rule that keeps firing on harmless content is a rule
+that will get the whole plugin switched off. Fixes, in order of preference:
+add the value to `allowlist` / `allowlist_substrings` in `policy.json`, narrow the
+rule, or add it to `disabled_secret_rules`.