Adopt silverassist/wp-plugin-kernel across all sub-plugins

[miguelcolmenares]

Summary

PR B1 of the standardization effort (see plan) — closes the wp-plugin-kernel rollout gap. This monorepo was the only SilverAssist WordPress plugin holdout still using a locally-duplicated bootstrap pattern instead of the shared silverassist/wp-plugin-kernel package (already adopted in 7 other plugins).

Changes

• Every LoadableInterface-implementing class in the 3 sub-plugins now implements the kernel's interface (get_priority(): int, should_load(): bool, init(): void) instead of the old local priority()/register() contract, plus a static instance(): self singleton method — AbstractPlugin::load_components() calls $class::instance() with no arguments.
• Core/Plugin.php in all 3 sub-plugins now extends AbstractPlugin, implementing only get_components(): array (a list of class-strings). Singleton access, priority-ordered loading, and per-component error isolation are inherited, not re-typed per plugin.
• contentful-tables' TableDataLoader was constructor-injected into ShortcodeRegistrar/SettingsPage before this — since kernel-managed components can't take constructor arguments, TableDataLoader became a proper singleton and the other two now fetch it via TableDataLoader::instance().
• graphql-shortcode-support had already independently converged on the same get_priority()/should_load()/init() method names before the kernel existed — its migration was just swapping the interface import and Plugin.php's base class.
• The 3 local Core/Interfaces/LoadableInterface.php files are deleted.
• silverassist/wp-plugin-kernel added to require (runtime, not --dev) in all 3 composer.json.

Docs

A pre-PR core-review pass caught 3 READMEs' directory trees and CONTRIBUTING.md's documented interface contract still describing the old local pattern — updated all 4.

Verified

• make phpcs, make phpstan, and make build all clean across all three sub-plugins
• Read-only core-review pass (read-only subagent): confirmed every migrated class has all 4 required singleton pieces, no orphaned references to the deleted interface files, contentful-tables' DI-to-singleton change didn't leave any stale new TableDataLoader() call sites, and composer.json changes are valid

Not done in this PR (see plan)

• Live functional verification in the Local Sites WordPress install — no automated test suite exists yet for any of the 3 sub-plugins (that's PR B2), so this PR relies on static analysis + CI's PHP 8.2/8.3/8.4 matrix rather than runtime testing. Recommended as a manual follow-up before merging, or accept CI as sufficient given the mechanical nature of the change.