feat(install): automate shell PATH integration

[Aaronontheweb]

Summary

Automatically make installed Netclaw binaries available on PATH while preserving an explicit opt-out:

• Unix: create ~/.netclaw/env and source it from the detected bash or zsh startup file; write a native fish conf.d snippet.
• Windows: persist the normalized install directory in the User PATH and update the current installer process without rewriting inherited Machine PATH entries.
• Support --skip-shell and -SkipShell for callers that do not want installer-managed PATH integration.
• Document the default behavior and opt-out commands.

Refactoring applied

This revision removes the mock-heavy shell-command tests and narrows the implementation around observable behavior:

• Shell paths are quoted safely, including spaces and apostrophes.
• Repeated installs are idempotent.
• macOS bash startup precedence and zsh's effective ZDOTDIR are respected.
• Unknown Unix shells are not modified and receive shell-neutral guidance.
• Windows User, Machine-inherited, and current-process PATH values remain separate.
• A stale Windows process PATH is repaired even when the User PATH already contains Netclaw.
• The dependency-free manifest parser uses portable POSIX tools, accepts reordered JSON fields, and selects assets from the requested release.

Independent adversarial hardening

A separate agent reviewed the implementation specifically for shell and PATH mutation hazards. Every actionable finding was fixed:

• Unix validates the physical path after symlink resolution and before copying binaries or modifying shell files, so a safe-looking symlink cannot introduce :, CR, or LF into persisted PATH data.
• zsh is queried after it reads .zshenv, so a non-exported ZDOTDIR is honored; unsafe or ambiguous results decline automatic mutation.
• Unsupported shells receive the literal directory and shell-neutral instructions rather than a Bash command that may be invalid.
• Windows PowerShell 5.1 no longer calls the modern-only Path.TrimEndingDirectorySeparator API; CI executes the complete smoke suite under both Windows PowerShell 5.1 and PowerShell 7.
• A literal %NAME% install path is preserved under REG_SZ and explicitly rejected before mutation under REG_EXPAND_SZ, where Windows would expand and corrupt it.
• The per-user HKCU\\Environment key is opened or created without elevation; the installer never modifies HKLM or the machine PATH.
• Existing raw User PATH text, REG_SZ/REG_EXPAND_SZ type, %VAR% references, inherited process entries, and empty-PATH semantics remain preserved.

Validation

• All 32 current GitHub checks pass on head 4da4e50a7.
• Unix installer smoke: 30/30 locally; Ubuntu and macOS CI additionally execute zsh/fish and physical-path coverage.
• Windows installer smoke: 24/24 under PowerShell 7 and 24/24 under stock Windows PowerShell 5.1, on both push and pull-request triggers.
• Full .NET suite: 6,013 passed locally with 14 opt-in integration tests skipped; all six Ubuntu/macOS/Windows CI test jobs pass.
• Native smoke passes on Linux and macOS for both trigger sets; screenshot regression and native publishing pass.
• shellcheck scripts/install.sh scripts/smoke/install-smoke.sh
• PowerShell parser validation
• dotnet slopwatch analyze: 0 issues
• pwsh -File ./scripts/Add-FileHeaders.ps1 -Verify
• git diff --check

Website documentation follow-up: netclaw-dev/netclaw-website#89.