docs: define telegram adapter migration

[maciek-hyperdev]

Summary

Defines the Telegram provider-adapter migration boundary for unified channels.

• records the current router as a compatibility primitive, not the final platform authority;
• defines provider mapping, single-poller, pairing, initiated-send and media requirements;
• adds DoR/DoD, parity gates, pilot rollout and rollback evidence for Agda then Calluna.

Why

Telegram is the first external-channel migration. Its existing direct MCP implementation must retain user-visible behaviour while provider configuration and routing move to the platform.

Validation

• git diff --cached --check
• Documentation-only change; no runtime behaviour changed.

Dependencies

Depends conceptually on hyperdev-platform PR #1232 and hyperdev-channels PR #18.

[maciek-hyperdev]

Closing this draft: it incorrectly mixes internal HyperDev architecture and rollout planning into the public project. A future public-facing proposal, if needed, will contain only generic, versioned adapter contracts and migration-safe compatibility guarantees.