Closes #972

Problem

has_frontmatter(), parse_frontmatter(), and remove_frontmatter() in src/basic_memory/file_utils.py detected frontmatter by substring/split (content.startswith("---") plus content.split("---", 2)) rather than by line-anchored fences.

Repro

A single-line string that merely starts with --- — where \n are literal backslash-n characters, a very common CLI/agent input shape:

bm tool write-note --title "Meeting Notes" --folder meetings \
  --content "---\nstatus: active\n---\nDiscussed Q3 roadmap with Anthony."

The CLI receives the literal one-line string ---\nstatus: active\n---\nDiscussed Q3 roadmap with Anthony.. The loose logic treated it as frontmatter:

1. yaml parsed \nstatus as a key, which got merged into the note's YAML and written to disk.
2. The body was silently transformed (the inline ---…--- segment was stripped).

Fix

Shape chosen: (b) anchor the parsing ourselves. A new shared _split_frontmatter() helper requires both fences on their own line (^---[ \t]*$); all three public helpers delegate to it, so detection is consistent.

Why (b) over delegating to python-frontmatter: the existing helpers raise ParseError with specific messages that callers and tests assert on ("Content has no frontmatter", "Invalid frontmatter format", "Frontmatter must be a YAML dictionary", "Invalid YAML in frontmatter"). Anchoring in-place keeps the diff minimal and preserves every one of those messages plus BOM stripping, empty-frontmatter -> {}, and non-dict -> ParseError.

The helper skips leading blank lines before the opening fence, so dedented heredoc-style content (a string beginning with a newline) still parses exactly as before — this does not relax line-anchoring, since the single-line repro's first line is not a bare ---.

All callers were reviewed (markdown/utils.py merge path, entity_service, sync_service, file_service, batch_indexer); none relied on the loose substring behavior.

Tests

• tests/utils/test_file_utils.py: line-anchored detection for the exact one-line repro, inline --- later in the first line, opening fence with no closing fence, and fences with trailing whitespace (valid); plus a pass-through regression confirming remove_frontmatter leaves the one-liner intact and parse_frontmatter raises rather than inventing a \nstatus key.
• tests/mcp/test_tool_write_note.py: integration-level regression through write_note — the literal one-liner is stored verbatim as the body and no garbage key leaks into the generated YAML frontmatter.
• All pre-existing frontmatter tests stay green.

Gates

• uv run pytest tests/utils/test_file_utils.py tests/mcp/test_tool_write_note.py — 90 passed
• caller suites (markdown, entity_service, sync, batch_indexer) — 159 passed
• uv run ruff check / ruff format --check on changed files — clean
• uv run ty check src — clean
• 100% coverage on changed lines

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