Add qdrant-edit tool for updating stored memories

[sascha08-15]

Summary

Adds a qdrant-edit tool that updates the closest matching memory in place, so stored information can be corrected without creating duplicate or stale entries.

• QdrantConnector.edit() — finds the closest matching point via semantic search, rewrites that point in place, and preserves existing metadata when replacement metadata is omitted
• qdrant-edit tool — registered alongside qdrant-store (write-mode only), reuses the same collection binding and filter ergonomics as qdrant-find
• TOOL_EDIT_DESCRIPTION — customizable tool description via environment variable, consistent with the existing tool description pattern
• Connector and settings coverage — adds integration tests for update behavior and settings tests for the new tool description
• MCP server surface coverage — adds tests that verify read_only exposure and default COLLECTION_NAME binding at the registered tool level
• README — documents the new tool and environment variable

Motivation

Issue #74 asks for edit/delete support so memories can be corrected instead of duplicated over time. PR #116 covers deletion; this PR covers the edit half by letting callers replace the nearest matching memory while keeping its point identity.

Test plan

• Runtime verification of connector edit behavior against local in-memory Qdrant
• Runtime verification of MCP tool registration behavior for read_only and default COLLECTION_NAME
• Manual MCP Inspector validation via @modelcontextprotocol/inspector --cli
• Full upstream pytest run in a normal networked environment

Inspector flow validated:

• tools/list exposes qdrant-find, qdrant-store, and qdrant-edit
• qdrant-store stores a memory successfully
• qdrant-edit updates a stored memory successfully
• omitting replacement metadata preserves the existing metadata on the updated memory
• qdrant-find returns the updated content and metadata after the edit
• read_only mode exposes only qdrant-find

Design decisions

• In-place update by point id — after semantic lookup, the existing point id is reused so the update does not create a second record
• Metadata preservation by default — if replacement metadata is omitted, the existing metadata is kept
• Same filter ergonomics as qdrant-find — the edit tool supports the same query filter flow and respects default collection binding
• Nearest-match semantics — there is no similarity threshold; the closest semantic match is updated, so query quality matters
• Write-mode only — registered inside the if not self.qdrant_settings.read_only: block, consistent with qdrant-store

[sascha08-15]

Manual MCP Inspector validation is done for the edit path. Verified through @modelcontextprotocol/inspector --cli that tools/list exposes qdrant-find, qdrant-store, and qdrant-edit; qdrant-store succeeds; qdrant-edit updates the stored memory; omitting replacement metadata preserves existing metadata; qdrant-find returns the updated content and metadata after the edit; and read_only mode exposes only qdrant-find.

One behavioral note from the same validation: qdrant-edit currently updates the nearest semantic match and does not apply a similarity threshold, so query quality matters.