Make agent file writes append by default

openspec/specs/meeting-summary/spec.md

@@ -68,7 +68,13 @@ The summary note SHALL keep frontmatter links to meeting artifacts and SHALL cop
 
 When the summary note already has an `attendees` frontmatter property, Meeting Assistant SHALL preserve the existing summary attendees instead of overwriting them from the meeting note.
 
-The summary agent SHALL be able to read and write the assistant context body as its own notebook using overwrite, replace, and insert line modes.
+The summary agent SHALL be able to read and write the assistant context body as its own notebook using append-by-default, explicit whole-body replacement, replace, and insert line modes.
+
+When the summary agent calls `write_context` without line edit arguments and without `replace_file`, Meeting Assistant SHALL append the supplied content to the assistant context body while preserving assistant context frontmatter.
+
+When the summary agent calls `write_context` with `replace_file` true, Meeting Assistant SHALL replace the assistant context body while preserving assistant context frontmatter.
+
+Meeting Assistant SHALL refuse ambiguous `write_context` calls that combine `replace_file` with line edit arguments.
 
 The summary instructions SHALL tell the summary agent to use transcript evidence and screenshot OCR participant evidence to refine the meeting note attendee list conservatively, adding or removing attendees only when the evidence is clear.
 
@@ -133,11 +139,25 @@ When `delete_identity` is called with an existing transcript speaker identity, M
 - **WHEN** the summary agent writes the summary or assistant context file
 - **THEN** Meeting Assistant does not include those writes in the external write diff audit
 
+#### Scenario: Assistant context body appends by default
+- **WHEN** the summary agent writes assistant context content without line arguments
+- **THEN** Meeting Assistant appends the content to the existing assistant context body
+- **AND** preserves the existing assistant context frontmatter
+
+#### Scenario: Assistant context body can be explicitly replaced
+- **WHEN** the summary agent writes assistant context content with `replace_file` set to true
+- **THEN** Meeting Assistant replaces the assistant context body
+- **AND** preserves the existing assistant context frontmatter
+
 #### Scenario: Assistant context body writes do not duplicate frontmatter
 - **WHEN** the summary agent writes assistant context content that accidentally includes a markdown frontmatter block
 - **THEN** Meeting Assistant preserves the existing assistant context frontmatter
 - **AND** writes only the replacement body content after that frontmatter
 
+#### Scenario: Assistant context write mode is ambiguous
+- **WHEN** the summary agent combines `replace_file` with `from`, `to`, or `insert`
+- **THEN** Meeting Assistant refuses the assistant context write
+
 #### Scenario: Summary agent refines attendees from clear participant evidence
 - **GIVEN** assistant context OCR states that visible meeting participants are complete
 - **AND** the OCR result names a participant missing from the meeting note