`session_store_sql` returns deleted sessions forever: no DELETE verb on `POST /agents/analytics/query` or `/agents/sessions/*`

The issue involves the session data pipeline and API routes. Start by examining the API routes listed in the issue, particularly /agents/analytics/query and /agents/sessions/* in the codebase. Look for where session data is written and queried, and understand the data flow from CLI to cloud storage. Check related issues #3811, #4094, #4939, #3777 for additional context. The goal is to add a DELETE route or cascade deletion from tasks to sessions.

Describe the bug

The session_store_sql tool with source: "cloud" keeps returning sessions
after they have been deleted everywhere the user can reach:

SELECT COUNT(*) FROM sessions

→ 482, while GET /agents/tasks returns 0 tasks.

These rows are served by POST https://api.githubcopilot.com/agents/analytics/query
and written by chronicle reindex via PUT /agents/sessions/{id},
POST /agents/sessions/{id}/events, and PUT /agents/sessions/{id}/logs.

Every one of those write routes is an upsert or append. No route accepts
DELETE, and POST /agents/analytics/query rejects any non-SELECT
statement. Once a row exists in the sessions table behind
/agents/analytics/query, no client operation can remove it.

OPTIONS against every route the CLI uses for session data:

DELETE appears on none of them.

Affected version

Copilot CLI 1.0.87 (Linux x64)

Steps to reproduce the behavior

1. Accumulate CLI sessions with remoteExport enabled (the default).
2. Delete the corresponding Mission Control tasks:
   DELETE https://api.githubcopilot.com/agents/tasks/{id} (the call the CLI's
   session picker makes), or via github.com/copilot/agent WebUI.
3. Delete all local session state under ~/.copilot/session-state/.
4. Set remoteExport to false.
5. Run session_store_sql with source: "cloud":

   SELECT COUNT(*) FROM sessions
   

Every previously-exported session is still returned. /chronicle and
cross-session search surface the same rows.

Expected behavior

Sessions deleted from github.com/copilot/tasks, from the CLI session picker,
and from ~/.copilot/session-state/ should stop being returned by
session_store_sql, /chronicle, and cross-session search.

Setting remoteExport: false should also provide some supported way to remove
data already exported — or the retention policy for that data should be
documented.

One of:

1. A DELETE route for rows in the sessions table — ideally bulk, e.g.
   DELETE /agents/sessions/{id} plus a "delete all my cloud session history"
   operation.
2. DELETE /agents/tasks/{id} cascading to the matching sessions row.

Additional context

Impact

1. Deletion is not honored. Session metadata, titles, and summaries remain
   queryable after deletion from every user-reachable surface.
2. remoteExport: false is not retroactive. It stops new writes but leaves
   every prior row in place, with no documented retention period.
3. No recourse exists through the CLI, the web UI, or the API.

Related issues

These look like the same gap — deletion never reaching the append-only
/agents/sessions/* → /agents/analytics/query pipeline:

Adding a DELETE route would likely resolve #3811 and #4094 as well.