feat(health): report a recall_log source that went quiet, not just a dead table

[MXAntian]

The gap

The existing stall check asks "has anything written recently" — only true once every writer is dead at the same time.

In practice they die one at a time. A refactor drops a CLI branch, that caller's recalls stop, the other callers keep the table busy, and the table reads healthy forever while the dead caller is invisible.

Two real instances, both found by hand weeks late

Both were sitting in recall_log the whole time as a source whose last_ts stopped on a date and never moved:

tool-recall-hook   2484 rows   2026-05-05 .. 2026-07-15   silent 20d
mcp                 668 rows   ...        .. 2026-07-10   silent 24d

What this adds

detectStalledSources() — any source with ≥ sourceStallMinHistory (200) rows and ≥ sourceStallDays (3) of silence.

The volume floor is what makes it usable: one-off CLI probes and debug labels live below it and would otherwise flood every run.

Verified against the real 21k-row table:

source	rows	silent	reported
tool-recall-hook	2484	20d	✅
mcp	668	24d	✅
context-builder	12083	0d	—
prompt-recall-hook	3051	0d	—
cli	23	21d	— (occasional caller, not a stall)

A stall is a signal, not a verdict. A renamed source looks exactly like a dead one from here, so the report says so and leaves the call to a human.

Which is the second half of this PR

Chasing the mcp entry showed it was a rename in effect. 3044bb2 dropped _source: 'mcp' from recall_memory's buildMemoryContext call:

-        _source: 'mcp',
-        _sessionId: null,

Those recalls kept happening — under the default 'context-builder' label. No traffic lost, but the by-source breakdown quietly stopped meaning what it says, and 668 rows' worth of MCP calls have been hiding inside another caller's 12k for 24 days. Label restored.

Verification

289 passed / 0 failed. Without the change the two new assertions go red while every whole-table assertion stays green — that gap is the bug:

✗ (d) a high-volume source that stopped is reported as stalled — []
✗ (d) stalled source carries volume and silence so it can be triaged

🤖 Generated with Claude Code