feat(cli): add --target flag to install and uninstall

[dale-stewart]

Closes #40.

Summary

• Adds --target <path> to nwave-ai install and nwave-ai uninstall, enabling installation to a non-default Claude config directory.
• Supports both real use cases from the issue thread: alternate global (nwave-ai install --target ~/.claude-nwave → run Claude with CLAUDE_CONFIG_DIR=~/.claude-nwave claude for a side-by-side config) and per-project (nwave-ai install --target ./.claude for dogfooding a fork without disturbing ~/.claude/).
• Pure Python — no shell scripts added, respects the existing zero-shell-scripts policy.
• Backward compatible: omitting --target is byte-identical to the previous release.

How it works

The flag is parsed in _handle_install / _handle_uninstall (manual parsing, consistent with how --yes and --density-only are handled), normalized via Path.expanduser().resolve(), and threaded into the install subprocess by setting CLAUDE_CONFIG_DIR=<absolute-target>. The existing PathUtils.get_claude_config_dir() seam already honors that env var, so no per-callsite plumbing was needed.

--target $HOME (or any path whose realpath equals realpath $HOME) is rejected at parse time with exit 2 and zero filesystem mutation — guards against the catastrophic global-overwrite case.

Load-bearing fix: hook-command path resolution

When nwave-ai install --target <non-default> runs, the existing installer correctly lands files under <target>/lib/python/des/, but des_plugin._generate_hook_command hardcoded lib_path = "$HOME/.claude/lib/python". At runtime, Claude Code passes hook commands to a shell where $HOME is the user's real home (not the chosen target), so the portable form pointed at the wrong directory — hooks failed or silently exercised the global install.

The fix is a 3-line conditional: when context.claude_dir == Path.home() / ".claude" keep the existing portable $HOME/.claude/lib/python form (preserves cross-machine ~/.claude sync semantics). Otherwise, emit the absolute <target>/lib/python. Non-default targets are per-machine by user choice, so the loss of cross-machine portability is the intended trade.

env.PATH already honored context.claude_dir (it computes str(context.claude_dir / "bin") at line 862), so no change was needed there — that was a happy accident from prior refactoring.

Files changed

File	Change
nwave_ai/cli.py	New _extract_target_flag helper; --target parsing in _handle_install; new _handle_uninstall wrapper; dispatch rewired (+79 lines)
scripts/install/plugins/des_plugin.py	3-line conditional + comment in _generate_hook_command (+13 lines)
tests/installer/unit/cli/test_target_flag.py	New: 10 tests covering arg consumption, env-var setting, ~ expansion, relative-path resolution, $HOME refusal (direct and via . from $HOME), uninstall symmetry
tests/plugins/plugin-architecture/unit/test_des_plugin_target_flag.py	New: 4 tests for hook-command target-aware path selection
tests/des/acceptance/test_hook_path_portability.py	Fixture monkeypatches Path.home() to tmp_path so the default-home assertions still hold under the test fixture

Net production: 2 files, +92 LOC. Tests: +282 LOC.

Test plan

• pytest tests/installer/unit/cli/test_target_flag.py tests/plugins/plugin-architecture/unit/test_des_plugin_target_flag.py — 14/14 green
• pytest tests/plugins tests/installer/unit/cli tests/des/acceptance/test_hook_path_portability.py tests/des/integration/test_hook_configuration.py tests/build/unit/shared — 275/275 green (2 skips are pre-existing container-level skips owned by tests/e2e/test_fresh_install.py)
• ruff check + ruff format --check on all modified files — clean
• mypy --ignore-missing-imports on modified files — clean
• Manual dogfood: nwave-ai install --target ~/.claude-la-nwave →
  • 149 skills, 32 agents (under agents/nw/), 5 DES bin shims, settings.json, templates landed in ~/.claude-la-nwave/
  • All 9 hook commands have PYTHONPATH=/home/<user>/.claude-la-nwave/lib/python (absolute, points at target)
  • env.PATH prepends /home/<user>/.claude-la-nwave/bin
  • ~/.claude/ mtime unchanged (zero global pollution)
  • nwave-ai uninstall --target ~/.claude-la-nwave --force removes all DES hooks from settings.json and the agents/ tree (residual files under bin/skills/scripts/templates/ are the known upstream issue nwave-ai uninstall leaves residual artifacts (skills, lib, DES hooks) #39, not a regression of this PR)
• nwave-ai install --target $HOME refused with exit 2 and no filesystem mutation
• nwave-ai install (no --target) byte-identical to today's behavior (verified by the existing test suite passing unchanged)

Open follow-ups (not in this PR)

• nwave-ai uninstall leaves residual artifacts (skills, lib, DES hooks) #39 (clean uninstall): the residue under <target>/bin, <target>/skills, etc. after uninstall --target is the same residue tracked upstream as nwave-ai uninstall leaves residual artifacts (skills, lib, DES hooks) #39. --target makes the problem visible in alternate locations too; the fix belongs in uninstall_nwave.py.
• ~/.nwave/global-config.json still lives at the user's real home regardless of --target. Density / attribution settings remain shared across all targets — typically desired, but worth noting.

Notes for review

The DESIGN considered (a) bash wrapper + curl distribution and (b) HOME=<target> override + post-install sed — both were rejected after maintainer feedback in the issue thread. CLAUDE_CONFIG_DIR is strictly better than HOME=<target> because it doesn't pollute the subprocess tree's view of $HOME for tools like git, gh, ssh, and pipx.

The hook-path conditional preserves the $HOME-portable form only for the default-home case, which is the one for which it was designed. Existing users who sync ~/.claude/ across machines are unaffected.