fix(framework): on-before-mergeable run stays on the session branch (#560)

[suleimansh]

Closes #560. Draft: the fix is a judgment call in a design area, flagging before it leaves draft.

The bug

The on-before-mergeable follow-up (fired after setReadyForMerge()) strands its output on a branch nothing merges:

* the-framework/surge-pricing-followup
  22be1cf  Record surge pricing decision and business knowledge   <- DECISIONS.md + KNOWLEDGE-BASE.md
  main: has neither file

So the #556 quality TODO entries and the #537 knowledge docs live only on a stranded branch. The next run branches from main and cannot read them, which defeats #537.

Why

The follow-up is spawned as a plain framework prompt child, so it gets the full #326 system prompt including the ### Session name step (commit dirty tree -> create the-framework/<name> -> checkout). It branches before writing anything, and writes its output on the new branch. That step is right for a session; the follow-up is a follow-up to one, not a session itself.

Fix

Spawn the follow-up --vanilla, which drops the built-in #326 prompt and with it the session-name step, so it stays on the session's current branch where its output rides to review and merge with the work. The follow-up prompt is self-contained (it names the TODO file and knowledge docs itself) and the always-on emit protocols remain.

Scoped to the on-before-mergeable spawn only. The maintainability/security preset spawns share spawnPromptRun but are left branching: each is a separate focused refactor that arguably wants its own branch/PR.

Alternatives / decision

• MVP (this PR): --vanilla. One flag, drops the whole built-in prompt.
• Targeted: drop only the ## Before starting changes section, keeping the rest of the built-in prompt. More precise, a little more plumbing. Happy to switch if you prefer keeping the rest of the System prompt #326 framing for the follow-up.

One thing to confirm: the intended landing spot is the session's current branch (so output merges with the work). Both #556 and #537 read that way; the eager-vs-after-merge setting is orthogonal (lands correctly either way on the session branch).

Tests

promptRunArgs gains a vanilla param; new test locks that the follow-up path carries --vanilla and a normal run does not. Framework suite: 542 pass / 0 fail, typecheck clean. Changeset: @gemstack/framework patch.