Make content optional in update-memory tool

[Copilot]

The update-memory tool required content even for tags-only updates, forcing users to fetch and re-provide unchanged content.

Changes

Tool schema (src/tools/update-memory/index.ts)

• Removed content from required fields; only hash is required
• Updated descriptions to clarify optional nature

Executor (src/tools/update-memory/executor.ts)

• Made content optional in UpdateMemoryArgs interface
• Fetch existing content when not provided: args.content ?? existing.content
• Enhanced messages to indicate update type ("tags updated only" vs "content and tags updated")

Tests (src/tests/memory-server-tests.ts)

• Updated testUpdateMemoryTags() to verify tags-only updates without content parameter

Usage

# Before: required content even for tags-only updates
update-memory --hash abc123 --content "unchanged" --tags "new,tags"

# After: content is optional
update-memory --hash abc123 --tags "new,tags"

Backward compatible - existing calls with content continue to work.

Original prompt

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This section details on the original issue you should resolve

<issue_title>update-memory tool</issue_title>
<issue_description>Tool does not allow a user to only update tags, it requires "content" even though no changes are required to it.
User should be able to update the tags in an efficient manner.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes update-memory tool #26

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