Bean File Operations

`beans_bean_file`

Single tool with operation selector:

read
edit
create
delete
update_frontmatter

Path semantics

Paths are resolved under .beans/
Leading .beans/ is accepted and normalized
Path traversal outside workspace bean root is blocked

`update_frontmatter`

Atomic metadata update without rewriting body.

Supported fields:

title, status, type, priority
parent_id, tags
blocking_ids, blocked_by_ids
pr, branch

Nullable fields can be removed by setting them to null.

Frontmatter normalization

title is normalized to double-quoted YAML scalar
Inline comments and YAML quoting edge cases are handled