fix: persist /model selection in topics whose session is missing or stale

[ryuhaneul]

Problem

In a topic that has no usable session yet, completing the /model wizard
reports success but the selection is persisted nowhere. Sessions are only
created by the message flow; the wizard's switch path records the target
only on an already-existing session object; and topic switches never touch
the global config by design. So in a brand-new topic (no message sent yet)
— or one whose session has gone stale via idle timeout, daily reset, or
the target provider's bucket reaching max_session_messages — the next
message resolves a fresh session from the global defaults and the user's
choice is silently discarded. Re-running /model doesn't help.

When the stored session was stale, the switch summary could also
advertise a "Resuming …" state belonging to a session the next message
was about to discard.

Fix

• New SessionManager.resolve_session_target(key, *, provider, model) -> (session, created): atomically (under the manager lock) record the
  target on the session the next message will actually use — fresh
  sessions are retargeted in place, stale or missing ones are replaced by
  a fresh shell carrying the target. Freshness is judged against the
  target provider's bucket count (plus the usual idle/daily checks),
  matching what the next message-flow resolve will decide.
• switch_model uses it for topic switches and recomputes the resume
  summary from the resolved session, so newly created or replaced shells
  no longer advertise stale resume state. DM/main-chat behavior is
  unchanged (global-config path).

Deliberately out of scope (pre-existing behavior, separate issue
candidates): the remaining unlocked full-file session writers
(resolve_session, reset_*) and update_session's caller-identity
write-back.

Tests

• E2E through the real message path: wizard ms:r callback → a normal
  turn with no override asserts the CLI AgentRequest carries the
  selected provider/model (fresh, empty-topic, and target-stale cases).
• Manager unit matrix: fresh retarget / stale shell / missing shell;
  current-fresh–target-stale and the reverse; deterministic two-order
  concurrency contract for the new method.
• Global config/file/CLI default unchanged on topic switches; DM
  no-session path creates nothing; provider bucket deep-snapshot
  preservation; stale-resume suppression; existing switch tests intact.

4 files changed, +438/−7. Rollback = revert (no config/schema impact).

Merge notes: #166 replaces the adjacent kill line in
switch_model (this PR's kill mocks adapt on merge). #164
(per-session effort) overlaps the same sync region — when it lands,
the creation path should pass reasoning_effort (preserving a fresh
session's effort when None). #156 moves bucket keys to
session_bucket_key(provider, interactive=…) — the target-count
lookup adapts accordingly.