fix(think): label channel speakers in messenger messages

[cjol]

This PR adds configurable speaker labels for @cloudflare/think messenger channel traffic and documents the default SpeakerName: text transcript shape. Fixes #1840.

Why

• Think already prefixes non-DM messenger messages before sending them to the model, but that transcript convention was not documented for agent authors.
• Smaller models can misread Name: text as an identity cue instead of speaker attribution, especially when the user message is short or the agent persona is strict.
• Direct messages do not need speaker labels because the conversation is already one-to-one. Keeping labels channel-only avoids adding identity noise where it does not disambiguate anything.
• The new API is formatter-only rather than a boolean plus name-source enum. Callers can keep the default cascade, choose a provider-specific field, change the shape, or return null to suppress a channel label for a specific author.

Public API Surface

Symbol	Kind	Notes
MessengerDefinition.channelSpeakerLabel	Option	Additive formatter for channel speaker labels.
ChannelSpeakerLabel	Type	Additive formatter type, returns string | null | undefined.
resolveChannelSpeakerLabel	Function	Additive helper exported from @cloudflare/think/messengers.
toMessengerUserMessage(event, channelSpeakerLabel?)	Function signature	Adds an optional formatter parameter while preserving existing calls.

Code Changes

• events.ts centralizes the channel speaker fallback cascade as fullName || userName || userId and treats null or an empty formatter result as label suppression.
• toMessengerUserMessage applies speaker labels only for non-DM threads, for both text messages and action events.
• chat-sdk.ts threads channelSpeakerLabel from each messenger definition into the model-facing message conversion path.
• The README and messenger docs now describe the default channel transcript format, the DM suppression rule, and the formatter hook.
• The changeset marks @cloudflare/think for a minor release because the messenger definition API gains a new option.

Compatibility

• Existing channel message formatting stays the same by default: SpeakerName: text using fullName || userName || userId.
• Regular direct messages remain unprefixed.
• Direct-message action events are now also unprefixed. If a consumer depended on action labels inside DMs, it should treat the conversation context as the speaker instead of parsing a prefix from the model-facing text.

[changeset-bot]

🦋 Changeset detected

Latest commit: 5f09f0c

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@cloudflare/think	Minor
@cloudflare/agent-think	Patch

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