Allow querying soft-deleted sessions via CLI and/or optional config

[msgongora]

Problem / motivation

Harnesses like Claude Code regularly prune or clean up older transcript files on disk. When agentsview sync reconciles provider roots and finds a file missing, it sets deletion_cause = 'source_missing' and populates deleted_at.

While AgentsView's local-first storage design intentionally preserves all messages, tool calls, and tokens in sessions.db, all standard CLI commands (session search, session list, session get, session messages) filter out records where deleted_at IS NOT NULL. This makes historical conversations invisible (from CLI and Web) even though the data remains intact in the SQLite archive.

Proposed solution

1. CLI Flag (--include-deleted):
   Add --include-deleted to agentsview session search and agentsview session list so CLI queries can optionally include tombstoned/soft-deleted sessions.
2. Config Entry (preserve_missing_sources):
   Add an optional setting in config.toml:

     preserve_missing_sources = true

   When enabled, reconciliation leaves missing files active instead of tagging them with source_missing tombstones.

Alternatives considered

No response