feat(session): add ts_before and breakpoint params to Session.messages API

[ryanwyler]

Summary

Adds two optional parameters to Session.messages() API for loading messages older than the initial 100-message window:

• ts_before: Unix timestamp - returns messages created before this point
• breakpoint: Boolean - when true, stops at the first compaction summary

Problem

Multiple open issues request the ability to access message history beyond the initial 100-message limit:

• [BUG] Cannot scroll to beginning of long conversations (message limit hardcoded to 100) (#6138) anomalyco/opencode#6137 - Cannot scroll to beginning of long conversations
• feature request: pagination for messages & sessions anomalyco/opencode#4918 - Feature request: pagination for messages & sessions
• old messages disappear anomalyco/opencode#7380 - Old messages disappear
• [FEATURE] Virtualized scrolling + paginated message loading for long sessions (#8535) anomalyco/opencode#6548 - Related pagination work

Current Session.messages() only supports a limit parameter, which returns the N most recent messages. There is no way to retrieve older messages.

Solution

This PR adds the foundational server-side API enhancement required by all proposed solutions. The implementation is minimal (8 lines of core logic) and non-breaking:

Core changes:

• packages/opencode/src/session/index.ts - Add params to Session.messages() schema and iteration logic
• packages/opencode/src/server/routes/session.ts - Expose params in HTTP API

Why this approach:

• Timestamp-based anchoring uses immutable reference points, eliminating state management complexity and race conditions that offset-based pagination would introduce
• Breakpoint support enables "load conversation history" (stop at compaction) vs "load full session" (ignore compactions) use cases
• Zero breaking changes - all parameters optional, existing functionality unchanged
• Prerequisite for all solutions - whether TUI, web client, or SDK consumers implement history loading, they all need this server capability

Usage

# Get messages before a timestamp
GET /session/{id}/message?ts_before=1768609939954

# Get messages before timestamp, stopping at first compaction
GET /session/{id}/message?ts_before=1768609939954&breakpoint=true

# Combine with limit
GET /session/{id}/message?ts_before=1768609939954&limit=50

Testing

Verified against sessions with 170+ messages and multiple compactions:

• ts_before correctly filters to messages older than timestamp
• breakpoint=true stops at first compaction part
• Combined parameters work together
• Existing behavior unchanged when params omitted