feat: add self-registering provider factory (v0.2.0)

[Wikid82]

Summary

Adds a self-registering provider factory registry, mirroring database/sql's Register/driver pattern, so new providers become discoverable by a consuming project without hand-edited switch statements.

type Factory func(config map[string]any) (Sender, error)
func Register(name string, factory Factory)
func New(name string, config map[string]any) (Sender, error)
func RegisteredTypes() []string

Each of the 8 existing providers (discord, slack, gotify, pushover, ntfy, webhook, telegram, email) now self-registers via init(). A new providers/all package blank-imports all of them for one-line full discovery. The existing typed constructors (discord.New(Config{...}, w) etc.) are unchanged and remain the primary, fully type-safe API — the registry is additive, not a replacement.

This is a breaking/additive change to the public API introduced in v0.1.0 (new exported identifiers + init-time side effects), so it ships as v0.2.0. v0.1.0 has not been adopted by anyone outside this project yet, so there's no compatibility concern.

Why map[string]any over json.RawMessage

email.Config carries non-serializable Go values (Mailer/TemplateRenderer interfaces, a TemplateName closure) that can never round-trip through JSON. A generic map keeps the registry usable for every provider, including that one.

Docs

• ARCHITECTURE.md (new): exactly how to add a new provider — package layout, the Sender contract, where the Register/init() call goes, providers/all wiring, test conventions. Written for a human contributor or a coding agent to follow without guessing.
• docs/INTEGRATION.md (new): Five Ws and One H integration guide for adopting this module in a project other than Charon.
• README: new section pointing at the registry pattern.
• CHANGELOG: retroactively split the old catch-all "Unreleased" entry into [0.1.0] (already shipped) and added [0.2.0] on top, so it's accurate going forward.

Validation

• go build/go vet/staticcheck: clean
• go test ./... -race: passing
• Coverage: 95.5% overall, no per-package regressions
• Zero github.com/Wikid82/charon/* imports (grep-confirmed)
• goreleaser release --snapshot --clean dry-run: succeeded

Downstream

Charon's adoption of this registry (replacing its hardcoded provider-construction switch with notify.New(...), while keeping its own explicit allowlist of which provider types it exposes) is in progress as part of Charon PR #1253, currently pinned to this branch via a local replace directive pending this merge + a real tagged release.

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