[FEATURE] Commands should traverse parent directories like CLAUDE.md does

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

Title: Commands should traverse parent directories like CLAUDE.md does

Description:

CLAUDE.md files are discovered by traversing up the directory tree, allowing teams to share configuration via a parent directory. However, .claude/commands/ are only discovered at the project level and ~/.claude/commands/ (personal), with no parent directory traversal.

This inconsistency makes it impossible to share custom commands across a team using the same pattern that works for CLAUDE.md.

Current Behavior

~/src/
├── .claude/commands/start-ticket.md    ← NOT discovered when in ~/src/project/
├── CLAUDE.md                           ← Discovered (traverses up) ✓
└── project/
    ├── .claude/commands/               ← Discovered ✓
    └── CLAUDE.md                       ← Discovered ✓

Expected Behavior

Commands should traverse parent directories and merge, just like CLAUDE.md:

• ~/src/.claude/commands/ (shared team commands)
• ~/src/project/.claude/commands/ (project-specific commands)
• ~/.claude/commands/ (personal commands)

Proposed Solution

Apply the same parent directory traversal logic used for CLAUDE.md to .claude/commands/ discovery. Commands found in child directories could override parent commands with the same name.

Alternative Solutions

1. Symlinks in every project - Each project symlinks .claude/commands/ to a shared location. Unacceptable maintenance burden.
2. Personal symlinks - Each developer symlinks shared commands into ~/.claude/commands/. Doesn't auto-update, requires manual maintenance per developer.
3. Git submodules per project - Each project includes shared repo as submodule with committed symlink. Works but requires setup in every repo.

None of these match the elegant CLAUDE.md solution where parent directory + symlink just works.

Priority

High - Critical for team adoption

Feature Category

CLI commands and flags

Use Case Example

Our team has 20+ repositories. We share workflows via a team-shared git submodule at ~/src/team-shared/. Currently:

• ~/src/CLAUDE.md symlinks to team-shared/CLAUDE.md → Works everywhere ✓
• ~/src/.claude/commands/ symlinks to team-shared/.claude/commands/ → Not discovered ✗

We want shared commands (/start-ticket, /pr-feedback) available in all projects without per-project setup.

Additional Context

Related issues requesting similar parent directory traversal:

• #12962 - Settings.json parent directory traversal
• #18029 - settings.json should traverse like CLAUDE.md
• #18192 - Recursive skill discovery (different but related)