feat: scaffold-seeded plan creation and a scaffold receipt contract

[A]

Groom now seeds a plan directory with one booping scaffold call instead of asking the model to transcribe a literal frontmatter block, and both writers that touch a plan's frontmatter answer with the same receipt.

Changes

• booping scaffold reports a unified diff per file it writes. A new file diffs against /dev/null; a file whose rendered content matches what is on disk produces no output. A rendered prompt can act on the receipt without reading the file back. difflib from the stdlib — scaffold's primary caller targets a vault that is not a git repository yet.
• Per-file write semantics replace the dest-is-not-empty precondition. An existing target is skipped and named, --force overwrites the files the tree names, and a destination already holding part of the tree exits 0 instead of erroring.
• booping frontmatter-update adopts the same stdout contract — the diff of the change it made, silence on a no-op — keeping its updated {path}: {keys} summary and errors on stderr, with arguments, flags and exit codes unchanged.
• frontmatter-update writes scalars with their YAML type. sp=23 lands as an integer, retro=null as a YAML null; a string whose plain form would reload as another type (yes) keeps its quotes.
• New core.groom_playbook.scaffold tree seeds index.md and an empty request.md, taking title and type from --set and created / commit from macros — so a plan carries the repo HEAD from the moment it is created rather than staying null until a sprint starts. Both --set variables render through default('') | tojson, so an omitted one cannot produce a null field or a broken document.
• The frontmatter shape lives in exactly one place. _partials/plan_frontmatter.md, _partials/plan_structure.md and docs/template_plan_frontmatter.md are deleted; the surviving template mechanics fold into _partials/plan_templates.md, which names the frontmatter-update invocation that writes the drafter's sp and summary.
• Scaffold documentation in documentation/project_config.md covers the third tree, the shared stdout contract, the skip rule and --force's narrowed meaning.

Review follow-ups

• The scalar-typing, frontmatter-update receipt and scaffold receipt suites are each one parametrized test over an input/expected table instead of one function per case. Assertions got stronger: scalar rows check the reloaded value's exact type, and the append and removal rows assert the full --- / +++ / @@ header. import yaml as pyyaml moved to module level.
• The code-review artifact and the two lessons the run produced are committed under vault/.
• The code-review exit hook parsed an inline code_reviews: value knowing only null and ~, so a plan carrying code_reviews: [] had that [] inserted as the list's first element. It now reads the flow form, and the script — previously untested — has a test module driving it as a subprocess.

Plan: vault/plans/202608091310_scaffold-seeded-plan-creation/.

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