feat(cli): accept WIREIT_WATCH env var as a --watch escape hatch (#1346)

[kiwigitops]

Resolves the short-term proposal in #1346.

Problem

npm/cli#8071 lands a warning today and will turn into a hard error in npm 12: npm run build --watch is rejected because --watch is not a recognized npm argument. Until now, wireit relied on npm's old behaviour of silently turning npm run build --watch into npm_config_watch= in the script's environment. After npm 12 there is no way to enable wireit watch mode from npm run — neither --watch (rejected) nor -- --watch (forwarded to the underlying script, not wireit).

@aomarks proposed in #1346:

1. Add a WIREIT_WATCH=1 environment variable.
   [...]
   I'm leaning towards doing 1 in the short term, because we already handle all other runtime wireit settings with WIREIT_ environment variables and it's very easy [...]

This PR implements proposal 1.

Change

Two getArgvOptions call sites now recognize WIREIT_WATCH (any non-undefined value) as an additional trigger for watch mode, OR'd with the existing detection:

   case 'npm': {
     return {
       watch:
-        process.env['npm_config_watch'] !== undefined
+        process.env['npm_config_watch'] !== undefined ||
+        process.env['WIREIT_WATCH'] !== undefined
           ? readWatchConfigFromEnv()
           : false,
       extraArgs: process.argv.slice(2),
     };
   }

And the parseRemainingArgs helper (used for yarn / yarnBerry / pnpm / nodeRun) gets a matching post-loop check so the escape hatch works on every agent, not just npm:

+  // Also accept WIREIT_WATCH as an escape hatch (matches the npm branch behaviour,
+  // useful for yarn/pnpm/node --run wrappers that scrub --watch from argv).
+  if (!watch && process.env['WIREIT_WATCH'] !== undefined) {
+    watch = readWatchConfigFromEnv();
+  }
   return {
     watch,
     extraArgs,
   };

The existing WIREIT_WATCH_STRATEGY / WIREIT_WATCH_POLL_MS env vars still control the watch strategy via the unchanged readWatchConfigFromEnv().

Test plan

• Added an ${agent} WIREIT_WATCH=1 run test case alongside the existing WIREIT_LOGGER env-var tests. Runs in the per-agent loop, so it covers npm, yarnClassic, yarnBerry, pnpm, and node --run in one go.
• All existing tests that exercise --watch and WIREIT_WATCH_STRATEGY are untouched and should continue to pass — the change is purely additive (an extra OR'd condition).

Notes for reviewers

• I deliberately scoped this to proposal 1 only. Proposals 2 (--wireit-watch arg) and 3 (declarative {"watch": true}) are bigger surface-area changes and can land on their own.
• WIREIT_WATCH= (empty string) also triggers watch mode, because the existing detection uses !== undefined rather than a truthy check — kept consistent so behaviour is predictable; matches how WIREIT_DEBUG_LOGGER is detected in logger.ts (via Boolean(...)) is intentionally NOT used here so an empty WIREIT_WATCH= set in CI's env still triggers, mirroring npm_config_watch=.