Support subfolder organization for slash commands

Problem

As the number of custom commands grows, the flat .claude/commands/ directory becomes hard to navigate. There's no way to logically group related commands.

For example, a team might have 15+ commands spanning development workflows, platform operations, and business processes. A flat list makes it harder to discover and manage them.

Proposed solution

Allow organizing commands into subdirectories inside .claude/commands/, with the folder name acting as a namespace prefix:

.claude/commands/
  dev/
    review-pr.md
    research-agents.md
    fix-e2e-tests.md
  platform/
    api-auth.md
    create-test.md
    create-issue.md
  ops/
    postmortem.md
    daily-summary.md
    generate-changelog.md

These would be invocable as /dev:review-pr, /platform:create-issue, /ops:postmortem, etc. The colon syntax is already used for skill namespacing (e.g., slack:summarize-channel), so this would be consistent.

Tab completion should also respect the grouping (typing /dev: would show only commands in that folder).

Current workaround

Prefix-based naming (dev-review-pr.md, platform-create-issue.md) partially solves discoverability but clutters filenames and doesn't provide real namespacing.

Prior art

• #2422 was closed as not planned, but the need has grown as command libraries get larger.
• Skills already support colon-namespaced invocation when provided by MCP servers.