[0/8] Stop a task write from dropping metadata or deleting the file

[alex-clickhouse]

Base of the task-board stack. Two defects in the task write path, both of which
lose user data. They sit below #272 because both are present on main today and
neither depends on the board work.

The metadata columns

upsert_task replaced every column on every call. A caller that rewrote a row
for some other reason — a file move, an appended note — had to read the row
first and pass source, source_url, deadline and tags back, or the write
nulled them.

Two bugs came from a caller that did not:

• TaskManager.reindex dropped tags after v018 added the column.
• task_done nulled all four when it moved a file into done/. fix(tasks): preserve task metadata on completion #238 fixed that
  one by adding the four lines back.

The nulled source_url also broke duplicate detection, because
_find_duplicate_tasks matches that column first. A completed task stopped
matching the source item that created it, so the next sync of that item made a
second task.

Those four columns are now preserve-on-omit: an omitted column keeps its stored
value. None still clears one, and "" still clears tags. Five call sites
stop restoring what they never change, and reindex stops merging file values
against the stored row by hand.

The second completion

The move wrote the destination and unlinked the source. Both are one path when
a task is completed twice, because the first completion stores a file_path
under done/ and the second one reads that path back as its source. The sequence
wrote the file and then deleted it. The task kept its row and lost its whole
markdown file, including the update history it had collected.

Reaching it needs nothing unusual. An agent that retries task_done after a
timeout sends the call twice, and with #272 the board can move a card out of
Done and back in. move_task_file now appends and renames, and a rename onto
one path is a no-op.

Tests

• test_task_upsert_preserve.py — the preserve-on-omit rule, its explicit
  clears, and the FTS text.
• test_task_completion.py — three tests for the second completion. All three
  fail on the old copy-and-unlink shape.

Full suite: 3030 passed.

🤖 Generated with Claude Code

[Copilot]

Copilot encountered an error and was unable to review this pull request. You can try again by re-requesting a review.