feat: add append param to update_drawer

[fkberthold]

Summary

Adds an append: bool = False parameter to tool_update_drawer (and its MCP
schema entry). When append=True and content is provided, the content is
concatenated onto the existing drawer body instead of replacing it, so a
caller updating a long drawer can send only the delta rather than
re-transmitting the whole body on every edit.

append defaults to False — existing callers and the replace-in-full
behavior are unchanged. The handler already fetches the existing document, so
this is a minimal read-modify-write with no extra round-trip:

new_doc = old_doc + sanitized if append else sanitized

Motivation and the alternatives considered (a separate append_to_drawer
tool, or a richer section-patch) are in the linked issue — this PR implements
the smallest form and is happy to follow a different surface if preferred.

Closes #1761

Changes

• mempalace/mcp_server.py
  • tool_update_drawer(...) gains append: bool = False; content path now
    appends-or-replaces based on the flag.
  • WAL log records the append flag.
  • mempalace_update_drawer MCP schema gains an append boolean property;
    tool description notes the delta-append behavior.
• tests/test_mcp_server.py
  • test_update_drawer_append — appends a delta and asserts the original body
    survives and the delta is concatenated (old_doc + sanitize_content(delta)).
  • test_update_drawer_replace_is_default — pins that the default
    (append=False) still replaces, guarding the back-compat path.

Verification

Test-first (RED → GREEN; visible in the commit order on this branch):

• RED commit adds test_update_drawer_append, which fails against the current
  code with TypeError: tool_update_drawer() got an unexpected keyword argument 'append'.
• GREEN commit adds the parameter; the test passes.

$ pytest tests/ -q
889 passed, 106 deselected, 1 warning in ~56s     # 1 warning is a pre-existing,
                                                   # unrelated fact_checker runpy warning
$ ruff check mempalace/mcp_server.py
All checks passed!

CONTRIBUTING.md adherence

• Conventional commits (test: for the failing test, feat: for the param).
• ruff clean on the changed file (100-char limit).
• Branched from and targeted at develop; pushed to a personal fork.
• Tests run without API keys or network access.

[gemini-code-assist]

Code Review

This pull request introduces an optional append parameter to the tool_update_drawer function, allowing users to concatenate new content to an existing drawer's body instead of replacing it. It also updates the tool's schema description and adds corresponding unit tests. A review comment points out a potential TypeError if the existing drawer content (old_doc) is None when attempting to append, and suggests safely defaulting it to an empty string.

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[gemini-code-assist]

If old_doc is None (which can happen if a drawer was created without a document or if the database is in an unexpected state), attempting to concatenate old_doc + sanitized will raise a TypeError.

To prevent this, we should safely default old_doc to an empty string if it is None.

Suggested change

	new_doc = old_doc + sanitized if append else sanitized
	new_doc = (old_doc or "") + sanitized if append else sanitized