fix(init): generated workflows invoke the diff CLI correctly

[lostmartian]

What

agentdiff init-generated workflows now call agentdiff diff <baseline> <candidate> --scenario <name> instead of the invalid agentdiff <candidate> --baseline <baseline> form. Adds a regression test.

Why

Found while rebuilding agentdiff-demo against 0.5.0-rc: with record/init/approve registered, the E1 diff-default patch prepends diff only when the first arg is not a command — the template form becomes diff <candidate> --baseline <baseline>, where --baseline is the baseline-store option, so the run dies with Missing argument 'candidate_path'. Every init-generated gate would fail in CI on first use.

How

• Both template steps (gate + PR report) switched to the explicit two-positional diff form.
• Regression test test_workflow_invokes_diff_with_valid_cli_form pins the exact invocation and bans --baseline from the generated workflow.

Testing

• make lint → All checks passed
• uv run python -m pytest → 430 passed (was 429)

Checklist

• Exactly one logical change in this PR
• make lint passes
• uv run python -m pytest is green
• Docs updated if user-facing
• CHANGELOG.md entry added under [Unreleased] (if feat/fix)
• Conventional title: type(scope): summary

Roadmap link: WS-E CLI / Phase 2 — 0.5.0 ship blocker found via agentdiff-demo PR #4.

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