Add qdrant-delete tool for memory management

[sascha08-15]

Summary

Adds a qdrant-delete tool that removes memories by semantic search, enabling cleanup of stale or contradictory entries. This addresses a gap where memories could be stored and retrieved but never removed, leading to accumulation of outdated information.

• QdrantConnector.delete() — finds the closest matching point(s) via embedding similarity and deletes them, returning the deleted entries for confirmation
• qdrant-delete tool — registered alongside qdrant-store (write-mode only, respects read_only setting), with collection_name partial application when a default is configured
• TOOL_DELETE_DESCRIPTION — customizable tool description via environment variable, consistent with existing TOOL_STORE_DESCRIPTION / TOOL_FIND_DESCRIPTION
• 5 integration tests — covering store-then-delete, nonexistent collection, empty collection, selective deletion preserving other entries, and custom collection deletion
• README — documents the new tool and environment variable

Motivation

Without a delete capability, stale memories accumulate and can contradict newer information. Users have no way to clean up outdated entries, which degrades the quality of semantic search results over time.

Closes #101, closes #74, closes #69

Test plan

• All 14 integration tests pass (9 existing + 5 new)
• Manual testing with MCP inspector (fastmcp dev)
• Verify read_only mode does not expose qdrant-delete
• Verify COLLECTION_NAME default applies to delete tool

Design decisions

• Semantic search for deletion — matches the existing UX pattern (users describe what to delete in natural language, same as qdrant-find)
• Default limit=1 — conservative default to avoid accidental mass deletion; callers can increase if needed
• Returns deleted entries — allows the LLM to confirm what was removed and inform the user
• Write-mode only — registered inside the if not self.qdrant_settings.read_only: block, consistent with qdrant-store

[sascha08-15]

Verified two previously unchecked test-plan items against the current delete implementation:

• QDRANT_READ_ONLY=1 exposes only qdrant-find, so qdrant-delete is correctly hidden in read-only mode.
• With COLLECTION_NAME configured, the registered qdrant-delete tool no longer exposes a collection_name parameter; without a default collection it remains exposed.

So the remaining unchecked item in this PR is the manual MCP inspector / fastmcp dev validation.

One more repo-management note: issue #101 is specifically the delete request, so this PR is the one that should close it on merge. That likely belongs in the PR body as Closes #101
rather than only in comments, if auto-closing is desired.

[sascha08-15]

fastmcp inspection done.

[sascha08-15]

Ready to go!