[FEATURE] .claudeignore — exclude files from Claude Code's context window

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

## Summary

Add support for a .claudeignore file that tells Claude Code to skip specified files and directories when building context, using the same syntax as .gitignore.

## Problem

Teams increasingly want to store non-code artifacts in their repos — security threat models, WIP design docs, ADRs, old specs — because version control and co-location with the code are genuinely valuable. But these documents create a real problem with Claude Code:
they get pulled into the context window during normal development sessions, where they can bias coding behavior, consume context budget, or surface stale/WIP information as if it were authoritative.

The current workaround — permissions.deny rules in .claude/settings.json — works but has poor discoverability and is unintuitive. Most developers won't know it exists or think to use it.

## Why Not Just Use permissions.deny?

• .claudeignore is immediately understandable to any developer who has used git
• It lives in the repo, so the exclusion is shared across the team automatically — no per-developer config needed
• permissions.deny is a hard block; .claudeignore is a softer "don't auto-consume" that still allows explicit access
• The mental model matches the use case: "ignore for context purposes" vs. "deny access entirely"

## Use Cases

• Security threat models and vulnerability assessments stored in docs/security/ — valuable to version but should not bias coding sessions
• WIP design documents that are incomplete and could mislead an AI assistant
• Historical ADRs that provide context for humans reading the repo but are too voluminous for routine coding context
• Generated documentation, changelogs, or reports that bloat the context window without adding coding value

Proposed Solution

A .claudeignore file at the repo root (and optionally in subdirectories) using standard .gitignore glob syntax:

Don't pull security docs into coding sessions

docs/security/

Ignore WIP specs

specs/wip/

Ignore old ADRs

docs/adr/deprecated/

### Behavior

• Files matching .claudeignore patterns are excluded from automatic context building
• Claude Code can still read them if the user explicitly asks (e.g., "read docs/security/threat-model.md") — this is not a permissions block, just a context filter
• Follows the same precedence model as .gitignore (directory-level files apply to their subtree)

Alternative Solutions

_No response_

Priority

Medium - Would be very helpful

Feature Category

CLI commands and flags

Use Case Example

_No response_

Additional Context

_No response_