[FEATURE] Disable and/or hide build in tools

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Currently there is no clear way to disable and/or hide the build in tools. The list of tools searchable via / contains so many tools which not useful in every project or situation but are loaded into the context.

Even if it is a good change to discover and invoke tools via the Skill tool which will only load them into context on demand the issue still persists that not every build in tool should be available (v2.1.108):
The model can now discover and invoke built-in slash commands like /init, /review, and /security-review via the Skill tool

The current strategy is really confusing. Sometimes a new tool is added without any way to disable it, sometimes a new tool and a config key is added but the key never mentioned in the change log and sometimes a new tool is added and its config key is mentioned in the change log.

Proposed Solution

Environment variables following the existing DISABLE_BUG_COMMAND=1 pattern:

DISABLE_SLASH_COMMANDS=/add-dir,/copy
HIDE_SLASH_COMMANDS=/install-slack

or

DISABLE_INSTALL_SLACK_COMMAND=1
HIDE_REVIEW_COMMAND=1

Alternative Solutions

Support two levels of command management via permissions config:

deny — Completely disables command (Claude cannot invoke it; users cannot access it)
{"permissions": {"deny": ["/add-dir", "/copy", "/risky-command"]}}

hide — Removes from UI for visual cleanup, but Claude can still invoke if needed
{"permissions": {"hide": ["/install-slack", "/quick-open", "/do-something"]}}

Priority

Medium - Would be very helpful

Feature Category

Configuration and settings

Use Case Example

Teams in restricted environments want to prevent risky operations (disable commands) while simplifying the UI without removing features Claude might use autonomously (hide less-common commands).
This mirrors how permissions.deny already works for tools/subagents extending the pattern to slash commands, skills and harness tools.

Additional Context

Issue #16758 was a similar idea.