✨ feat: add uv workspace composability and project discovery

[renardeinside]

Summary

Implements #90 — make apx composable via uv workspaces.

• Add apx init --as-member [MEMBER_PATH] flag (defaults to packages/app) to scaffold an app as a uv workspace member
• Auto-detect member mode when apx init runs in a directory with an existing pyproject.toml without [tool.apx]
• Add find_app_dir with automatic project discovery (scans subdirectories up to 5 levels deep) so all commands (dev start, build, check, etc.) work from the workspace root
• Add shared modify_pyproject helper to eliminate TOML read/parse/write duplication between ensure_apx_uv_config and ensure_workspace_config
• 25 unit tests covering discovery, workspace config, depth limits, idempotency

Changes

File	What
crates/cli/src/common.rs	find_app_dir, discover_apx_project, has_apx_config, modify_pyproject + 12 tests
crates/cli/src/init.rs	--as-member flag, auto-detection, ensure_workspace_config, refactored ensure_apx_uv_config + 5 tests
11 command files	Switch from resolve_app_dir to find_app_dir (one-line change each)

Zero changes to crates/core — all new logic is self-contained in CLI.

Closes #90

Test plan

• cargo test -p apx-cli — 25 tests pass
• cargo clippy -p apx-cli — clean
• just check — all checks pass (ruff, prettier, cargo check, clippy, fmt, ty)
• cargo test --workspace --exclude apx-studio — full suite passes
• Manual: apx init --as-member in empty dir creates workspace layout
• Manual: apx init in dir with existing pyproject.toml auto-detects member mode
• Manual: apx dev check from workspace root discovers nested app

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