Add assignee/assignees filter fields to GitHub triggers

[geoffjay]

Summary

Add optional assignee and assignees filter fields to the existing GitHub trigger source types (github_issues and github_pull_requests). This brings GitHub triggers to parity with the new GitLab triggers and the existing Linear assignee filter.

Context

The issue description for #1178 explicitly notes:
"Note that assignee was added to the issues trigger and assignees to the merge requests, this is an optional field that I want to add, and I've actually been meaning to add to GitHub as well."

Currently, GitHub triggers fetch all matching issues/PRs by labels and state but have no assignee filtering. The gh CLI supports --assignee for filtering.

Acceptance Criteria

• Add assignee: Option<String> to TriggerConfig::GithubIssues in crates/orchestrator/src/scheduler/types.rs
• Add assignees: Option<Vec<String>> to TriggerConfig::GithubPullRequests in crates/orchestrator/src/scheduler/types.rs
• Both fields use #[serde(default)] for backward compatibility
• Update GithubIssueSource::new() to accept optional assignee parameter
• Update GithubIssueSource::fetch_tasks() in crates/orchestrator/src/scheduler/github.rs:
  • When assignee is set, add --assignee {value} to gh issue list args
• Update GithubPullRequestSource::new() to accept optional assignees parameter
• Update GithubPullRequestSource::fetch_tasks():
  • When assignees is set, add --assignee {value} for each assignee (gh CLI supports repeated --assignee)
• Update create_source() in crates/orchestrator/src/scheduler/runner.rs to pass new fields through
• Update SourceTemplate::GithubIssues and SourceTemplate::GithubPullRequests in crates/cli/src/commands/apply.rs with the new fields and conversion
• Update source_github_issues and source_github_pull_requests in schemas/workflow.yml with new optional properties
• Update existing tests in github.rs to verify assignee field handling
• Add test for assignee filtering in gh CLI args construction

YAML Example

# GitHub Issues with assignee filter
source:
  type: github_issues
  owner: myorg
  repo: myrepo
  labels: [agent]
  assignee: alice
  state: open

# GitHub PRs with assignees filter
source:
  type: github_pull_requests
  owner: myorg
  repo: myrepo
  assignees: [bob, carol]
  state: open

Key Files

• crates/orchestrator/src/scheduler/types.rs - add fields to enum variants
• crates/orchestrator/src/scheduler/github.rs - implement filtering
• crates/orchestrator/src/scheduler/runner.rs - pass fields to constructors
• crates/cli/src/commands/apply.rs - SourceTemplate updates
• schemas/workflow.yml - schema updates

Stack Base

Stack on: feature/autonomous-pipeline
Parallel: no ordering constraint (independent of GitLab work, touches only GitHub code)