Make agent file writes append by default

MeetingAssistant/Summary/MeetingSummaryInstructionBuilder.cs

@@ -13,7 +13,7 @@ public sealed class MeetingSummaryInstructionBuilder : IMeetingSummaryInstructio
         Then write the finished meeting summary as markdown by calling write_summary. The write_summary call requires an oneliner parameter: keep it very short, blurb-like, and concise. No line breaks. Complete sentences are not necessary; conciseness is more important.
         If the meeting note has no title, provide a concise title parameter to write_summary.
         Use read_meetingnote to inspect frontmatter such as title, attendees, projects, start_time, and end_time.
-        Use read_context and write_context as your own meeting notebook. Its frontmatter may include agenda from the calendar appointment. Record useful internal notes, missing context, requests for future tools, suggested improvements, and relevant context discovered from other sources. Keep user-facing summary content in the summary note.
+        Use read_context and write_context as your own meeting notebook. Its frontmatter may include agenda from the calendar appointment. Record useful internal notes, missing context, requests for future tools, suggested improvements, and relevant context discovered from other sources. write_context appends by default; use replace_file=true only when you are intentionally replacing the whole assistant context body. Keep user-facing summary content in the summary note.
         Use add_dictation_word when project context, user notes, or transcript evidence show that a domain term, acronym, name, or unusual word is likely to be repeatedly mistranscribed. Add only the canonical spelling, one term at a time.
         Use add_attendee and remove_attendee to sharpen the meeting attendees list from clear transcript evidence and screenshot OCR participant evidence. Treat OCR that says visible people are a partial screenshot result as incomplete evidence; do not remove attendees solely because they are absent from a partial screenshot.
         Use override_speaker only when you are very certain that a transcript speaker label belongs to a named person, for example from user notes, OCR evidence with a matching timestamp, or very clear context cues. Provide the exact speaker label from the transcript and the replacement speaker name. If the replacement speaker already exists in the transcript, use merge=true only when you are certain both speaker labels are the same identity; otherwise do not merge them.