Task 0.1.4: CLI Context Management (`attach`, `detach`)

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Task 0.1.4: CLI Context Management (attach, detach)

Title: Implement dataset context attachment for CLI sessions

Description:
The CLI should maintain a "current context" (attached dataset) that persists across commands within a session. This enables workflows like: attach to orders table → ask questions → see lineage → export results.

Why: Without context, every command requires repeating dataset identifiers. Context management makes the CLI feel like a proper development environment.

Acceptance Criteria:

• dataing ds attach <dataset_id> sets current context
• dataing ds attach <urn> resolves URN to dataset
• dataing ds attach --sql "SELECT..." --datasource <id> attaches from SQL
• dataing ds detach clears context
• dataing ds current shows current context with details
• Context persists in ~/.dataing/context.json
• All investigation commands use current context if no explicit target
• Context includes: dataset_id, datasource_id, URN, schema snapshot
• --no-context flag on any command ignores saved context

Key Design Notes:

• Context file is JSON with schema version for forward compatibility
• URN resolution uses GET /api/v1/asset-instances/search
• SQL attachment creates virtual context with inferred schema

Key APIs:

• GET /api/v1/asset-instances/search (exists)
• GET /api/v1/datasources/{id}/schema (exists)

Dependencies:

• None (can be built independently)

Risks + Mitigations:

• Risk: Stale context after schema changes → Mitigation: Warn if schema changed since attachment

Effort: S (2-3 days)

Designation: OSS