docs: refresh README for 0.5.0rc2 surface

[cipher813]

Summary

README had drifted to the 0.2.0-era surface. Pulls it forward to cover everything we shipped across 0.5.0rc1 + rc2.

Badges

• Tests 243 → 376
• PyPI v0.2.0 → v0.5.0rc2
• New: PEP 561 typed marker

Lead with the new surface

• Quick Start now shows FlowDoctor.builder() + TelegramNotifierConfig instead of the (now @deprecated) flow_doctor.init() yaml flow.
• All three pipeline-integration idioms (guard / monitor / direct report) on the builder-constructed instance.
• report_async() + flow_doctor.context() contextvars sections.
• flow_doctor.testing pytest plugin + RecordingFlowDoctor section — downstream test ergonomics, no imports required.
• FlowDoctorProtocol contract for type-checked swap-in.
• flow_doctor.otel.report_to_otel_span_event adapter section.

Telegram becomes the recommended default

• Five-notifier table (Telegram / Slack / Email / GitHub / S3) replaces the old four-line "Notifications" bullet list.
• Setup recipes pointing at @BotFather and the /getUpdates lookup.
• Env-var quickstart leads with FLOW_DOCTOR_TELEGRAM_*.

Env-var table additions

• FLOW_DOCTOR_TELEGRAM_BOT_TOKEN + FLOW_DOCTOR_TELEGRAM_CHAT_ID
• FLOW_DOCTOR_S3_BUCKET (was missing entirely)
• FLOW_DOCTOR_SKIP_PREFLIGHT (documented opt-out for tests / offline boot)

YAML + migration

• YAML config example refreshed to lead with telegram + s3 entries.
• New "Migrating from 0.4.x" section showing the PEP 702 @deprecated migration path; yaml shim keeps working through the 0.5.x series.

Install / dev

• Install commands carry --pre during the rc cycle; one-line explainer that the flag drops away once 0.5.0 final ships.
• Architecture diagram updated with the three subpackages added in this release: _protocol.py, testing/, otel.py, py.typed.
• Tests count in the Development section bumped 212 → 376.

Test plan

• pytest tests/ → 376/376 pass (no code changes; docs-only).
• GitHub markdown renderer sanity — no broken anchors / mis-rendered code fences (will visually verify on the PR page).

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