feat(channels): canonicalize conversation bindings

[AllenReder]

Summary

Canonicalizes channel conversation_key generation through a host-owned runtime helper and updates channel adapters to use route keys for Telegram, Slack, Mattermost, Matrix, Email, and Webhook fallback conversations.

This is an intentional breaking change for existing durable channel bindings: old key shapes such as telegram:123 are not migrated or looked up. Existing external conversations start fresh unless the operator explicitly resumes the intended session.

Behavior Changes

• Telegram private chats now bind as telegram:dm:<chat_id>; non-private chats bind as telegram:group:<chat_id> without topic routing.
• Slack and Mattermost channel messages bind to channel-level keys, while explicit thread roots bind to :thread:<id> route keys.
• Matrix, Email, and Webhook fallback keys are canonicalized and URL-encode route ids; explicit Webhook conversation_key values still win.
• Channel /resume now persists the current external conversation binding to the resumed session, so the next inbound message in that conversation continues in the resumed transcript.

Documentation

• README or docs updated for user-facing behavior, CLI/configuration, runtime layout, provider behavior, security policy, package behavior, state/versioning behavior, or test/gate workflow changes.
• Documentation update is not needed because:

Updated developer delivery/context docs and the channel configuration how-to, including the zh-CN site mirror.

Verification

List the commands you ran and the result.

uv run pytest tests/runtime/test_session_routing.py tests/runtime/test_session_commands.py tests/runtime/test_sessions_context.py tests/channels
# 188 passed

uv run python -m compileall demiurge/runtime demiurge/channels tests/runtime tests/channels
# passed

git diff --check -- demiurge/channels demiurge/runtime tests/channels tests/runtime docs/developer-guide/delivery-runtime.md docs/developer-guide/runner-and-context.md docs/how-to/configure-channels.md website/i18n/zh-CN/docusaurus-plugin-content-docs/current/developer-guide/delivery-runtime.md website/i18n/zh-CN/docusaurus-plugin-content-docs/current/developer-guide/runner-and-context.md website/i18n/zh-CN/docusaurus-plugin-content-docs/current/how-to/configure-channels.md
# passed

Change Checklist

• The PR is scoped to one feature, fix, cleanup, or documentation pass.
• Dependencies were not added or changed, or the dependency impact is explained above.
• Breaking changes are absent, or they are called out in the summary.
• Secrets, API keys, tokens, and private local paths are not included.
• Generated assets are updated when source assets changed.

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