Keep multi-line agent notes intact through emit and parse

[fohara]

Replaces #37, which GitHub closed when its base branch was deleted on merge of
#36. Same change, rebased onto main.

What was wrong

lash add --agent-note "$(printf 'first\nsecond')" wrote
@agent-note: first followed by a bare, unindented second. The parser
treats an unindented line as the end of the annotation block, so lash show
afterwards returned only "first" and the rest of the note was gone, exit code 0.

I had previously assessed this as unreachable from the CLI on the grounds that
notes are single-line. That was wrong; the shell one-liner above is all it
takes.

format_task_annotations built the note with a single format! and pushed the
result as one "line", so any newline in the value landed in the file with no
indentation.

The fix

The emitter writes the first line as the @agent-note: line and each
continuation with the annotation indent, which is the shape
parser::annotations::parse_annotation_block folds back into one value.

Indentation alone is not enough for every value, because the folding is lossy
in two ways. A blank line is skipped, so a note containing one comes back with
the gap closed up. A line starting with @ is read as the start of a new
annotation, truncating the note and possibly inventing an annotation nobody
asked for. MarkdownEmitter::check_agent_note rejects both at validation time
with E_CREATE_INVALID_AGENT_NOTE, naming the offending line, and leaves the
file untouched.

Leading whitespace on a continuation line is normalized rather than rejected.
The parser trims it, so indentation is lost but no text is, and rejecting it
would rule out ordinary things like an indented list inside a note.

Tests

A round-trip property test over notes of one to four lines at four indent
levels, emitting through format_task_annotations and reading back through
parse_annotation_block. Unit tests for the accept and reject sets and for
continuation indentation. Three integration tests covering a two-line note
surviving through to lash show, and both rejection cases leaving the file
byte-identical.