fix: stop --append-agent-note duplicating @agent-note past body text

[fohara]

Fixes #74.

Root cause

TaskLines::find_annotation_line in update/mutations.rs searched only the contiguous annotation block directly under the checkbox, stopping at the first non-@ line. The parser is more forgiving: an @key: line found after free-text body lines becomes an orphaned annotation and is merged into the most recent task (parse_task_section_internal). So a note below body text belonged to the task in the parsed model, but the editor couldn't see it — --append-agent-note concluded the task had no note and fell through to the create path, writing a second @agent-note: under the checkbox. Lint then rejected the file as a duplicate annotation, lash index skipped it, and later mutations died on E_INDEX_STALE.

Fix

• find_annotation_line now scans the task's whole body region — up to the next checkbox line or ## heading, the same boundary that ends the parser's orphan merging.
• continuation_range handles the orphaned position with a stricter rule (only lines indented deeper than the annotation), so trailing same-indent body text is never swallowed.
• append_agent_note indents the continuation relative to the found annotation line (unchanged result for the normal in-block case).

This also fixes the same latent duplicate-insert path in --agent-note (replace), --owner/--estimate, and @labels: edits.

Tests

• Unit tests: append/replace past body text stays a single annotation, lookup doesn't reach into the next task, same-indent trailing body text isn't swallowed.
• Integration test reproducing the issue verbatim: append lands as a continuation, file stays lint-clean and indexable, and a second append keeps working (no E_INDEX_STALE).

Out of scope

The parser attaches an orphaned annotation as a single line and silently drops its indented continuation lines, so lash show renders only the first line of a note in this position. That predates this fix (the repro's original "Second line" was already invisible) and is a lash-core parsing gap worth its own issue.