refactor: default /ba:plan to decisions, gate code behind a shape label

[azevedo]

What

/ba:plan no longer mandates literal implementation code in every plan. Plans now default to decisions — approach, exact file paths, patterns to follow, pseudo-code for shape, and test scenarios. A real code block is permitted only under a **Code-shape decision:** <why> label, where the code's shape is the design decision.

This reframes plan code from rework-prevention (unmeasured, and already half-disabled by /ba:execute) to design validation — a forcing function applied precisely where the design is non-obvious.

Why

Reviewing /ba:plan docs is expensive because the implementation body — code plus comments — is 60–80% of every painful plan, not the framing sections (consolidated verbosity research on this branch). A provenance check found the keep-code rule is inherited ce-plan heritage that ce itself reversed six days after our fork. Dropping it where it doesn't pay removes the bulk of review cost without losing the one place code earns its keep.

The contract change (the ~45 lines that matter)

The source change is small; most of this diff is the research → brainstorm → plan trail that motivates it.

• plan.md — templates and "Key rules" default to decisions; the **Code-shape decision:** trigger ships with a positive/negative example pair + a when-unsure tiebreaker; Step 0 and the rule anchor code-bearing blocks to the brainstorm's ## Locked Design.
• execute.md — Step 2b implements to the plan's decisions, while any literal code block stays binding-when-present (backward-safe: pre-change plans, which are all literal code, execute unchanged). Step 1.5b gains a literal-vs-pseudo discriminator so the scope-tripwire isn't biased by pseudo-code.
• slice.md — same discriminator + pre-change backward-compat in LoC sizing.
• README.md, CLAUDE.md (label-sync convention across the four files), plugin.json (0.23.0 → 0.24.0).

Backward compatibility

Plans already on disk are full literal code with no labels. The rule "any literal code block is binding when present" means they execute and slice exactly as before — no version flag, no migration.

Out of scope / deferred

• /ba:review-plan restructure — deferred; the hybrid changes its inputs, so its fate is decided after this lands.
• The internal code-repo research pass (company data) is not included — kept local.
• Manual validation (re-render a STANDARD + COMPREHENSIVE plan under the new rule) is noted in the plan but not yet run.

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