fix: enforce workspace isolation to prevent agent pollution (#57) (#72)

Problem: After installing Agent Reach, agents may clone repos, create
files, or write output into the workspace directory, gradually polluting
the user's project. Over time this degrades the agent experience.

Fix:
- SKILL.md: add Workspace Rules section — never create files in workspace
- install.md: add Directory Rules table, explicit boundary about workspace
- install.md: all git clone commands now target ~/.agent-reach/tools/
- update.md: add workspace warning
- cli.py: install creates ~/.agent-reach/tools/ directory

All agent-facing docs now enforce: /tmp for temp, ~/.agent-reach/ for
persistent data, workspace stays untouched.

Co-authored-by: Panniantong <panniantong@users.noreply.github.com>

docs/update.md

@@ -12,6 +12,10 @@ Copy this to your AI Agent:
 
 ## For AI Agents
 
+### ⚠️ Workspace Rules
+
+**Never create files, clone repos, or run commands in the agent workspace.** Use `/tmp/` for temporary work and `~/.agent-reach/` for persistent data.
+
 ### Goal
 
 Update Agent Reach to the latest version, then verify everything still works.