feat(morning-brief): inline brief prompt into task description

[harshithmullapudi]

Summary

• Remove the "Morning Brief" skill document from the default seed. The full brief prompt now lives directly on the recurring task's page via createScheduledTask({ description }), so the agent has the instructions on every firing without a skill-lookup indirection.
• Initial scope is Gmail only — GitHub and Calendar sections are stubbed in the prompt for later. Slack send + the exact <h3>Brief — ...</h3> scratchpad-update structure are preserved.
• Backfill: missing tasks are created with the description; existing tasks with an empty page get the description written; tasks with a non-empty description are left alone (avoids clobbering customizations).

Files

• apps/webapp/app/services/morning-brief.ts (new) — exports MORNING_BRIEF_TASK_DESCRIPTION.
• apps/webapp/app/models/workspace.server.ts — drops the skill gate, seeds the task with the description.
• apps/webapp/app/scripts/backfill-morning-brief.ts — drops skill ensuring; backfills descriptions for legacy seeds.
• apps/webapp/app/services/skills.defaults.ts — removes the Morning Brief entry from DEFAULT_SKILL_DEFINITIONS.

Out of scope (call out)

• Existing morning-brief skill documents in already-seeded workspaces are not deleted. They become redundant but harmless. Happy to add a soft-delete step in the backfill if we want a clean slate.
• The update_task description-guard for recurring tasks is unchanged — seeding bypasses it via direct createScheduledTask and the backfill writes via setPageContentFromHtml. A separate change can add an editPrompt: true opt-in to update_task if needed.

Test plan

• Create a fresh workspace and confirm the seeded "Morning brief" task has the Gmail-only description on its page.
• Run MODE=morning WORKSPACE_IDS=<id> tsx apps/webapp/app/scripts/backfill-morning-brief.ts against a workspace whose morning-brief task currently has an empty page; verify the description is written.
• Re-run the backfill and confirm it reports skipped: task ... already has a description.
• Run the backfill against a workspace with no morning-brief task; confirm seeded: task id=... (created with description).
• Wait for a 9am firing (or manually trigger one) and confirm the brief is delivered over Slack AND appended to today's scratchpad under the exact <h3>Brief — ...</h3> block.

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