fix(cli): stabilize timeline json schema

[upsetbit]

Summary

Fixes #299.

Timeline --json now emits a stable snake_case CLI schema instead of raw Go struct field names, and the documented jq example now treats timeline output as NDJSON.

What changed

• Added a timeline JSON DTO with fields like id, agent, device_id, project, and started_at.
• Added snake_case token usage fields under usage.
• Reused the same JSON emitter for local and remote timeline output.
• Preserved remote profile, parent_session_id, and kinds in remoteSessionToLocal.
• Updated tests and the docs/usage.md jq example to use NDJSON correctly.

Validation

• go test ./internal/cli -run 'TestRunNuRemoteListsWithoutLocalStore|TestTimelineSessionPayloadUsesStableJSONSchema' -count=1
• go test ./internal/cli/... -race
• go build -o ./bin/prosa ./cmd/prosa
• pnpm exec markdownlint-cli2 --no-globs docs/usage.md
• Manual checks with prosa --all --json --limit 1 | jq '{id, ID, agent, Agent, started_at, StartedAt, project, profile}'
• Manual check of the updated jq -s '[.[] | {agent, project, started_at}]' example