Add plugin support and centralize config resolution in VS Code extension

[clavery]

Summary

Stacked on #174.

Plugin support for the VS Code extension

The b2c-cli plugin system (keychain managers, vault integrations, etc.) was only available through oclif hooks. The VS Code extension called resolveConfig() directly, bypassing all installed plugins. This adds a new SDK module (@salesforce/b2c-tooling-sdk/plugins) that handles plugin discovery and loading without depending on @oclif/core, making it reusable by the extension, MCP server, or any non-oclif consumer.

New SDK module (packages/b2c-tooling-sdk/src/plugins/):

• discovery.ts — Discover installed plugins by reading the oclif data directory
• loader.ts — Hook context shim + dynamic import of hook files (with new Function workaround for esbuild CJS bundling)
• manager.ts — B2CPluginManager orchestrator: invokes b2c:config-sources, b2c:http-middleware, and b2c:auth-middleware hooks with same priority mapping as the CLI
• index.ts — Barrel file + ./plugins subpath export

Extension integration (packages/b2c-vs-extension/src/plugins.ts):

• Lazy singleton that initializes B2CPluginManager during activation (fire-and-forget, non-fatal)
• getPluginConfigSources() returns { sourcesBefore, sourcesAfter } for config resolution

Centralize config resolution

Replace 7 independent resolveConfigWithPlugins() call sites with a single B2CExtensionConfig singleton that lazily resolves, caches, and exposes getConfig() / getInstance() / getConfigError() / reset().

• Add dw.json file watcher (RelativePattern + onDidSaveTextDocument) that auto-resets config and fires onDidReset event
• WebDAV tree subscribes to onDidReset for automatic refresh on config changes
• Silently handle FileNotFound in tree re-expansion after server switch (no error toasts for missing paths)
• Delete WebDavConfigProvider (fully replaced by B2CExtensionConfig)
• Add resolveForDirectory() for the deploy-cartridge case that uses a different working directory

Test plan

• pnpm --filter @salesforce/b2c-tooling-sdk run test:agent — plugin discovery, loader, manager tests
• pnpm --filter b2c-vs-extension run typecheck:agent && pnpm --filter b2c-vs-extension run build
• pnpm run lint:agent
• Reload extension with plugins installed — verify Output > B2C DX shows plugin discovery
• Open SCAPI explorer — verify prefill populates from config (including plugin-provided credentials)
• Edit dw.json hostname — WebDAV tree should auto-refresh (check [Config] log lines)
• Switch server in dw.json while tree is expanded — no error toasts for missing paths

[clavery]

Superseded by #186