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.

.gitignore

@@ -0,0 +1,11 @@
+# ctxguard runtime state lives in ~/.claude/ctx-guard/ and must NEVER be committed.
+# These entries are a second line of defence in case anything is copied in by hand.
+**/entities.json
+**/policy.json
+**/index.json
+**/audit.jsonl
+**/.ctxguard-cache/
+*.local.md
+__pycache__/
+*.pyc
+.venv/