Stop Auto Memory from retrying low-signal sessions indefinitely

## Problem

Auto Memory only records a candidate session as processed when the extraction
agent successfully reads the transcript with `read_file`. If the agent sees a
session in the index and decides not to read it because it looks low-signal,
that session remains unprocessed and can be surfaced again in future runs.

Relevant code:

- `packages/core/src/services/memoryService.ts`

The current flow tracks processed sessions through `processedSessionKeys`, which
is populated from successful `read_file` activity. Sessions that are surfaced to
the agent but intentionally skipped are recorded as candidates, but not as
processed.

## Impact

- Low-signal sessions can repeatedly consume background extraction runs.
- The 30-minute throttle reduces frequency but does not solve repeated work.
- Backlog rotation helps, but repeated retries still make the service less
predictable and more expensive over time.

## Expected Behavior

Auto Memory should distinguish between:

- sessions successfully inspected via `read_file`
- sessions intentionally skipped as low-signal
- sessions that failed to read and may need retry

Low-signal skipped sessions should not keep resurfacing as new candidates on
every extraction run.

## Acceptance Criteria

- Candidate sessions can be recorded as skipped or attempted even if not read.
- Repeated low-signal sessions do not keep resurfacing every extraction
interval.
- Failed reads remain distinguishable from intentional skips.
- Tests cover a candidate session that is surfaced but not read by the
extraction agent.

## Notes

This is primarily a background reliability and cost-control issue. The goal is
not to force the extractor to read more sessions, but to persist a terminal or
backoff state when it chooses not to inspect a candidate.