fix(tasks): order board and list reads newest-first, and report the window

[bkearns]

task_board and task_list silently omitted recently created tasks.

Four tasks created hours earlier were absent from both, while task_get returned them in full — so an agent that captures a deferral and reads the board back cannot see its own write, which looks like the write failed. /whats-next and /roadmap both depend on the board being the durable record.

Cause

The limit was applied by CQL with no ordering. The table is PRIMARY KEY (tenant_id, task_id), so rows arrive in task_id order and a LIMIT took an arbitrary slice rather than the newest rows. With a few hundred open tasks the window stopped including anything recent.

CQL cannot fix this with ORDER BY — created_at is not a clustering column. So ordering has to happen after the fetch, which means fetching enough to order meaningfully and applying the caller's limit afterwards.

Changes

• Reads fetch to MAX_FETCH_ROWS, sort newest-first, then apply limit/offset.
• Ties break on task_id, so the order is total and a page boundary cannot return one task twice and another never.
• list_tasks_paged reports total and truncated. A capped read that reports itself is usable; one that does not is worse than an error.
• offset added to task_list for paging a long board.

Also: the board exceeded the token limit

Separately from the invisibility, task_board returned 619,574 characters for 382 tasks and exceeded the tool-result token limit outright, so it could not be read at all without spilling to a file first.

Rows are now slim by default — body, result and metadata dropped, summary kept because a one-line summary is what makes a listing decidable. full: true restores everything, and task_get was always the way to get detail for the task you actually pick.

CLI

frg task list keeps emitting a plain array on stdout, because frg task list | jq depends on it. It gains the ordering fix, and truncation is warned on stderr so a capped read still says so without breaking the pipe.

Verification

84 test blocks pass, clippy clean. New tests cover newest-first ordering, tie-breaking on id, and that slim drops the bulk while keeping the decidable fields.

Verified against a live board — same query, before and after:

before:  08-14 08:19  t_000d6754   (first row)
after:   08-17 14:40  t_d91e9d83   (first row)

Not merging this myself: standing rule is no auto-merge on public repos.