fix(generator): don't double braces in a review prompt that never reaches format()

[tend-agent]

Problem

generate_review is the only generator that emits the prompt inside a GitHub Actions expression, and it escapes braces for format() unconditionally — including on the path where format() is never used.

_escape_braces doubles every brace that isn't the {pr_number} placeholder. That is correct going into format(), which collapses each {{ / }} pair back to a single brace. But generate_review only wraps the result in format() when the placeholder was actually found:

format_body, needs_format = _escape_braces(raw_prompt, "pr_number")
escaped = format_body.replace("'", "''")
if needs_format:
    prompt_expr = f"format('{escaped}', github.event.pull_request.number)"
else:
    prompt_expr = f"'{escaped}'"     # <- doubled braces, nothing to collapse them

An adopter whose workflows.review.prompt contains braces but no {pr_number} gets the doubled pairs written straight into a bare string literal. Reproduced against main (fd23cd0) with prompt: "Review this PR. Use the {code-review} skill.":

${{ 'Review this PR. Use the {{code-review}} skill.' }}

Nothing downstream collapses that pair — format() is what does the collapsing and it isn't in the expression. So the agent receives {{code-review}} where the adopter wrote {code-review}, and the doubled }} also lands inside a ${{ … }} expression, which is not a shape the generator should be emitting either way.

Only review is affected: every other workflow renders its prompt as a plain YAML block scalar (block_prompt in macros.yaml.j2, and the literal bodies in ci-fix.yaml.j2 / mention.yaml.j2), so no brace escaping is involved.

Solution

Return the prompt untouched when the placeholder isn't present. Escaping is a format() concern, so it now happens only on the branch that produces a format() call.

Testing

Two tests in test_generate.py pin the two branches — braces preserved without the placeholder, braces doubled and {pr_number} → {0} with it. The first fails on main:

assert '{{generated}}' not in "${{ 'Review...rated}}.' }}"
  '{{generated}}' is contained here:
    ${{ 'Review this PR. Skip files matching {{generated}}.' }}

test_prompt_with_numbered_placeholders in test_config_edge_cases.py pinned the old output and had to change — worth a look, since it's a test asserting the defect rather than an incidental snapshot. Its rationale was "escaped to prevent format() runtime errors", but its fixture ("Fix issue {1} and {2}") has no {pr_number}, so no format() call is generated and there is no runtime error to prevent. It now asserts the prompt is emitted verbatim. The sibling test_prompt_with_zero_placeholder, whose fixture does carry {pr_number}, is unchanged and still passes.

Full generator suite: 387 passed. ruff check / ruff format --check at the pinned v0.14.11 are clean.

Scope

No generated file in this repo changes — tend's own .config/tend.yaml sets no custom review prompt, and the default (/tend-ci-runner:review {pr_number}) carries the placeholder, so it takes the format() branch as before. The behavior change is visible only to an adopter with a brace-carrying custom review prompt, whose next regen will drop the spurious doubling.