feat: add CLAUDE_NOTIFIER_DISABLE env var for per-session opt-out

[ashmitb95]

What & why

Closes #63.

Hooks are installed globally and run for every Claude session on a machine. On a shared SSH host this means the host's hooks play sounds for other users' sessions, with no per-session way to opt out — the existing mute flag is machine-wide and persistent.

This adds an opt-in CLAUDE_NOTIFIER_DISABLE environment variable. When set in a shell (to any value other than empty/0/false), every hook exits silently — no sound, popup, or signal — for sessions in that shell only. An SSH user adds export CLAUDE_NOTIFIER_DISABLE=1 to their session/rc and their sessions go quiet; other sessions and the machine-wide mute flag are unaffected.

Env vars propagate from the shell → claude → hook child processes, so this is the natural per-session scope. This is the same mechanism the reporter prototyped by patching _lib.ps1 directly — this PR makes it official and update-safe.

How

• New helpers mirroring the existing isMuted short-circuit:
  • isDisabled() in hook/_lib/config.js
  • Test-NotifierDisabled in hook/_lib.ps1
• Gated as the first statement in all 12 hooks (6 JS + 6 PowerShell), so a disabled session exits before any signal write or playback.

Backward compatibility

With the var unset, isDisabled() returns false and every hook runs byte-for-byte as before. The check is pure (reads one env var, no I/O). No extension/src change — the long-running extension can't see a per-session env var, and the SSH/terminal-fallback path is exactly where the problem lives.