Define trajectory JSON schema v0

[rmems]

Goal

Define the first JSON schema for software-engineering trajectory records.

Context

The core data unit should describe one useful engineering trajectory, such as:

issue/review signal → before state → patch/fix → validation → outcome

Tasks

• Add schemas/pr_trajectory.schema.json.
• Add fields for repository, PR number, source URL, language, domain, task type, review signal, before context, after patch, validation events, outcome, and training use.
• Add enum-like conventions for task_type, outcome, and training_use.
• Add a tiny example record under datasets/examples/.

Proposed top-level fields

• id
• repo
• pr_number
• source_urls
• language
• domain
• task_type
• issue_context
• review_signals
• before_context
• patch
• validation
• outcome
• training_use
• quality_score

Acceptance Criteria

• Schema validates the example JSON file.
• Example is small enough to commit safely.
• README documents this as schema v0, not final.