feat(bootstrap): empty bootstrap - core services become explicit

[tgiachi]

Summary

The bootstrap no longer auto-registers the default services. From this change, SquidStdBootstrap.Create registers only the configuration core - DirectoriesConfig, the logger config section and the config manager (new RegisterConfigServices). Everything else is explicit.

Note for reviewers: this is intentionally a behavioral break for Create consumers, released as a minor (0.15.0). Commits deliberately carry no BREAKING CHANGE marker to avoid a 1.0.0 bump.

API

• RegisterCoreServices() (parameterless) - services only: JSON serializer, event bus, job system, main-thread dispatcher, timer wheel, metrics, secrets. Call it after Create (e.g. bootstrap.ConfigureServices(c => c.RegisterCoreServices())) or pick individual RegisterX() methods. It no longer re-registers a config manager with default name/dir.
• RegisterCoreServices(configName, configDirectory) - unchanged net behavior (config core + services) for standalone containers.
• New SquidStdBootstrap.Create(Action<SquidStdOptions>) - also fixes the latent bug where the dotnet new templates used this overload but it did not exist.
• The storage config section moved from the core defaults into AddFileStorage() (SquidStd.Storage now references SquidStd.Abstractions); RegisterDefaultCoreConfigSections removed (each RegisterX owns its section, logger lives in the config core).

Consumers updated in the same pass

• All samples call RegisterCoreServices() explicitly (runtime-verified: GettingStarted, EventsJobsScheduling, Messaging, Caching, Database, Templating, Vfs, Crypto, WorkerSystem, ScriptingLua, Secrets, Storage, AspNetCore). Two pre-existing sample bugs fixed along the way (Secrets ISecretProtector clash workaround, Storage YAML record).
• All 4 dotnet new templates updated (#if template directives untouched).
• Docs: landing quick start, tutorials, DI/bootstrap concepts with a "Migrating to 0.15" section, Services.Core/AspNetCore/Templating READMEs.

Follow-ups (non-blocking, out of scope)

• RegisterKmsSecretProtector/RegisterAwsSecretsManagerStore should use IfAlreadyRegistered.Replace so they override the defaults cleanly.

Test Plan

• 5 new EmptyBootstrapTests (config-core-only, explicit completion + start, logging in empty mode, standalone overload unchanged, Create(Action))
• Full suite 1004/1004; build 0 errors; docfx 0 warnings
• Runtime sweep of the samples; templates syntax-checked
• Guard: no !/BREAKING CHANGE markers in any commit (semantic-release must produce 0.15.0, not 1.0.0)