Optimize initial session load summaries

[mistercrunch]

Summary

• add an initial_summary sessions query path backed by selected SQL columns instead of full session rows
• type the UI initial session store as InitialSessionSummary, omitting large/deferred fields like tasks, full description, and full custom_context
• expose a capped first_prompt_preview for initial-load display/search and fetch full session data on demand for settings/template flows
• add initial-load byte instrumentation for ?debugLoad=1

Notes

• The DB column remains description for compatibility, but initial-load consumers now use the derived first_prompt_preview field.
• tasks is intentionally omitted from the initial summary type rather than filled with [], so accidental full-session assumptions are caught by TypeScript.
• genealogy and git_state remain in the summary because the tree/fork/SHA UI still depends on them.

Validation

• pnpm i
• pnpm check
• pnpm --filter @agor/core test -- sessions.test.ts --run
• pnpm --filter agor-ui test -- useAgorData.test.tsx --run