feat(core): store block metadata as YAML frontmatter in .md files (#9365)

* feat(core): store block metadata as YAML frontmatter in .md files

Block .md files in git repos now embed metadata (description, limit,
read_only, metadata dict) as YAML frontmatter instead of a separate
metadata/blocks.json file. Only non-default values are rendered.

Format:
  ---
  description: "Who I am"
  limit: 5000
  ---
  Block value content here...

Changes:
- New block_markdown.py utility (serialize_block / parse_block_markdown)
- Updated all three write/read paths: manager.py, memfs_client.py,
  memfs_client_base.py
- block_manager_git.py now passes description/limit/read_only/metadata
  through to git commits
- Post-push sync (git_http.py) parses frontmatter and syncs metadata
  fields to Postgres
- Removed metadata/blocks.json reads/writes entirely
- Backward compat: files without frontmatter treated as raw value
- Integration test verifies frontmatter in cloned files and metadata
  sync via git push

🐾 Generated with [Letta Code](https://letta.com)

Co-Authored-By: Letta <noreply@letta.com>

* fix: derive frontmatter defaults from BaseBlock schema, not hardcoded dict

Remove _DEFAULTS dict from block_markdown.py. The core version now
imports BaseBlock and reads field defaults via model_fields. This
fixes the limit default (was 5000, should be CORE_MEMORY_BLOCK_CHAR_LIMIT=20000).

Also:
- memfs-py copy simplified to parse-only (no serialize, no letta imports)
- All hardcoded limit=5000 fallbacks replaced with CORE_MEMORY_BLOCK_CHAR_LIMIT
- Test updated: blocks with all-default metadata correctly have no frontmatter;
  frontmatter verified after setting non-default description via API

🐾 Generated with [Letta Code](https://letta.com)

Co-Authored-By: Letta <noreply@letta.com>

* fix: always include description and limit in frontmatter

description and limit are always rendered in the YAML frontmatter,
even when at their default values. Only read_only and metadata are
conditional (omitted when at defaults).

🐾 Generated with [Letta Code](https://letta.com)

Co-Authored-By: Letta <noreply@letta.com>

* fix: resolve read_only from block_update before git commit

read_only was using the old Postgres value instead of the update value
when committing to git. Also adds integration test coverage for
read_only: true appearing in frontmatter after API PATCH, and
verifying it's omitted when false (default).

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Co-Authored-By: Letta <noreply@letta.com>

* test: add API→git round-trip coverage for description and limit

Verifies that PATCH description/limit via API is reflected in
frontmatter after git pull. Combined with the existing push→API
test (step 6), this gives full bidirectional coverage:
- API edit description/limit → pull → frontmatter updated
- Push frontmatter with description/limit → API reflects changes

🐾 Generated with [Letta Code](https://letta.com)

Co-Authored-By: Letta <noreply@letta.com>

---------

Co-authored-by: Letta <noreply@letta.com>

letta/server/rest_api/routers/v1/git_http.py

@@ -494,6 +494,8 @@ async def _sync_after_push(actor_id: str, agent_id: str) -> None:
                     logger.exception("Failed to read repo files after %d retries (agent=%s)", max_retries, agent_id)
 
         expected_labels = set()
+        from letta.services.memory_repo.block_markdown import parse_block_markdown
+
         synced = 0
         for file_path, content in files.items():
             if not file_path.startswith("memory/") or not file_path.endswith(".md"):
@@ -501,12 +503,20 @@ async def _sync_after_push(actor_id: str, agent_id: str) -> None:
 
             label = file_path[len("memory/") : -3]
             expected_labels.add(label)
+
+            # Parse frontmatter to extract metadata alongside value
+            parsed = parse_block_markdown(content)
+
             try:
                 await _server_instance.block_manager._sync_block_to_postgres(
                     agent_id=agent_id,
                     label=label,
-                    value=content,
+                    value=parsed["value"],
                     actor=actor,
+                    description=parsed.get("description"),
+                    limit=parsed.get("limit"),
+                    read_only=parsed.get("read_only"),
+                    metadata=parsed.get("metadata"),
                 )
                 synced += 1
                 logger.info("Synced block %s to PostgreSQL", label)