feat(doctor): detect when jaw is unreachable from a non-interactive shell (#479)

[parkjs101]

What

jaw doctor now reports when jaw cannot be resolved from a non-interactive shell — the condition behind #479, which is invisible from the shell doctor itself runs in.

Why this was invisible

ssh host 'jaw serve ...' runs a non-login, non-interactive shell that reads none of the files scripts/install.sh writes to:

Shell	Installer writes to	Read by ssh host 'cmd'
bash	.bashrc, .bash_profile	✗ — .bash_profile is login-only, and Debian's stock .bashrc returns on its first line when non-interactive
zsh	.zshrc, .zprofile	✗ — non-interactive zsh reads only .zshenv
other	.profile	✗ — login-only

So ~/.local/bin never reaches PATH, nohup fails to resolve jaw, and the only evidence is No such file or directory — a message that never mentions PATH.

A naive check could not catch this either: doctor runs in the interactive shell where PATH already works. The check therefore asks the question against getconf PATH.

Verification

$ jaw doctor
⚠️  Non-interactive PATH (ssh): jaw is in /Users/syon/.nvm/versions/node/v22.19.0/bin but not on the
   non-interactive PATH — remote `ssh host 'jaw ...'` will fail with "No such file or directory"
     call it by absolute path: …/bin/jaw
     or export PATH in the remote command: …
     or (zsh) add to ~/.zshenv: …

$ env -i PATH="$(getconf PATH)" sh -c 'command -v jaw'
(nothing — the warning is accurate)

$ env -i PATH="$(getconf PATH)" sh -c 'export PATH="…/bin:$PATH"; command -v jaw'
…/bin/jaw          # the printed remedy works

Running it live also caught a real defect before merge: the first version searched only a static list of install dirs, so a version-managed Node (nvm/fnm/Homebrew) read as "not installed" and the check stayed silent on a host that is broken. Candidate dirs now include the caller's resolved npm prefix and binary dir.

11 unit tests (tests/unit/noninteractive-path.test.ts), tsc --noEmit clean, full suite shows 0 new failures vs an origin/dev baseline.

Refs #479

Stack (merge bottom-up):

#	Layer	Review focus
4	docs — remote/headless guide	docs only
3	supervisor backend	generated sh + liveness contract
2	service guidance	operator message correctness
1	doctor detection ← you are here	detection logic + probe seam

Review this PR's diff only; its base is the layer below.