feat(CON-001): declare compat manifest + wire gate CI

[mrnicholasbcarter-code]

Summary

• Adds .verdict/compat-manifest.json, an exact copy of the CON-001 compat manifest (manifest_hash sha256:a608860555863e94af19efb28568d7d3abac87ea0c88c79bf5464f9e8346ef43, schema_version "1").
• Wires a compat-gate CI job in .github/workflows/ci.yml that installs verdict-core and runs verdict compat check --declared .verdict/compat-manifest.json --json against the declared manifest.

Dependency — CI will stay red until this merges

This PR depends on unmerged verdict-core PR #290 (feat/con-001-compat-gate, commit 7291941), which introduces the verdict compat manifest|check CLI this gate relies on. The new compat-gate CI job installs verdict-core directly from that PR's commit as a temporary pin:

pip install "git+https://github.com/mrnicholasbcarter-code/verdict-core.git@7291941"

This pin is temporary and clearly marked with a YAML comment in the workflow. CI on this PR is expected to stay red/fail until verdict-core PR #290 merges — at that point the pin should move to @main or a released version tag.

Test plan

• pytest — 18 passed
• ruff format --check . — passed
• python -m build — sdist + wheel built successfully
• git diff --check — no whitespace errors
• ruff check . / mypy src/ — both have pre-existing failures on master (unrelated to this change: an unused import in provider_receipts wiring, and a missing numba stub in the local env); not touched by this PR's 2-file diff
• compat-gate CI job — will fail until verdict-core PR #290 merges (expected, see above)

DO NOT MERGE until verdict-core PR #290 merges and the pin above is updated.

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