docs(opstack): experimental is pushed and tracked -- correct the flat "never push" rule

The rule said "Never push" without qualification, which was true while
experimental existed only locally and became misleading the moment it did not:
a future session would read it and let origin/experimental drift.

Now: push experimental freely (tracked since 2026-07-29), never push main, which
stays pinned at the known-good commit. Both statements of the rule are updated
(OPSTACK-HANDOFF.md and OPSTACK-PROMPT.md's crash-safe discipline).

Also added at both sites, because it is the failure mode this creates: a pushed
commit is NOT a "verified green" marker. This branch carries unbuilt work by
design, so OPSTACK-PROGRESS.md remains the only record of what was actually
built and measured, and the remote records only what was written.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

OPSTACK-PROMPT.md

@@ -146,7 +146,9 @@ mid-edit, and nobody will be watching.** Everything below follows from that.
 CRASH-SAFE DISCIPLINE (non-negotiable when unattended)
 1. `git commit` after every coherent unit — a file, a test, a header. Small and often. You are on
    branch `experimental`; `main` is the known-good fallback, so committing costs nothing and a
-   half-finished commit is infinitely better than an uncommitted half-edit. Never push.
+   half-finished commit is infinitely better than an uncommitted half-edit. Pushing `experimental`
+   is fine and expected (it is tracked as `origin/experimental` since 2026-07-29); **never push
+   `main`.**
 2. Maintain `OPSTACK-PROGRESS.md` at the plugin root. APPEND (never rewrite) a dated entry per
    milestone: what you did, what you believe is true, what is UNVERIFIED (i.e. everything not yet
    built), and the single next action. Write the entry BEFORE starting the work it describes, so an