Add vip_governance__deny_message filter for custom block-deny snackbar

[smithjw1]

Description

Closes #136.

When a block is restricted by governance rules, Gutenberg's block inserter shows a generic "Block 'X' can't be inserted." snackbar that gives editors no context — non-technical users interpret it as a bug. This PR adds a JS filter, vip_governance__deny_message, that integrators can use to replace that copy with something meaningful (e.g. who to contact).

How it works

• A new subscription on core/notices watches for Gutenberg's inserter-notice snackbar (dispatched in @wordpress/block-editor's use-block-types-state.js when an inserter selection is denied).
• When seen, the plugin parses the block title from the localised message (regex built from __() so it follows the active locale; supports both %s and numbered %1$s placeholders), resolves the matching blockName via the blocks store, runs everything through the new vip_governance__deny_message filter, removes WP's snackbar, and dispatches our own.
• If no integrator filter is registered, a sensible default is shown: "The 'Audio' block is restricted by your site's governance rules."
• The filter signature mirrors the existing vip_governance__is_block_allowed_for_insertion filter — (message, blockName, blockTitle, governanceRules).

Docs

• New vip_governance__deny_message section added to README.md (Code Filters) with signature and an integrator example.
• Added a row to the filters table in AGENTS.md.

Open question for the team

Should we also add a denyMessage field to the governance rules JSON schema (per-rule, alongside allowedBlocks/blockSettings), so non-technical admins can configure the copy without writing JS? The filter shipped here lets integrators override programmatically; a schema field would complement it for the no-code case mentioned in the issue. Happy to do as a small follow-up PR if there's appetite.

Steps to Test

1. Check out PR.
2. npm install && npm run build.
3. wp-env start and log in at http://localhost:8889 (admin/password).
4. Open a new post. In the browser console, dispatch the same payload Gutenberg dispatches when a block is denied:

   wp.data.dispatch('core/notices').createErrorNotice(
     `Block "Audio" can't be inserted.`,
     { type: 'snackbar', id: 'inserter-notice' }
   );

5. Verify the snackbar reads "The 'Audio' block is restricted by your site's governance rules." (and Gutenberg's original inserter-notice is gone — wp.data.select('core/notices').getNotices() should not contain it).
6. Register a filter and re-dispatch:

   wp.hooks.addFilter(
     'vip_governance__deny_message',
     'example/custom',
     (msg, blockName, blockTitle) => `Reach out to #content if you need "${ blockTitle }".`
   );
   wp.data.dispatch('core/notices').removeNotice('wpcomvip-governance-deny');
   wp.data.dispatch('core/notices').createErrorNotice(
     `Block "Audio" can't be inserted.`,
     { type: 'snackbar', id: 'inserter-notice' }
   );

   Verify the snackbar reflects the override.
7. npm run test:js — 33 tests pass.
8. wp-env start && npx playwright install chromium && npx playwright test --grep "deny-snackbar|vip_governance__deny_message" — both new E2E tests pass.

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