Provider/plan switch meter reload and wave-loop monitor integration

[Ryan-Atkinson87]

Capstone for the Usage monitor milestone: handle mid-project provider/plan switching, and wire
the assembled monitor into the live wave loop.

Spec: §6.8 (Provider/plan switching), §6.6 (surface which meter is governing), §4.2 (the
wave loop), §18.7.

Acceptance criteria

• On a provider/plan switch (from config or the UI mid-project), the monitor reloads the correct meter set (Usage meter model, governing-meter selection, and reader interface #30 plan→meter-set mapping) and the reader selects the active path (Subscription usage reader: OAuth /api/oauth/usage poll with degradation chain #31 subscription / API-mode usage reader from response headers (built, inactive) #32 API). No restart required.
• Integration: the monitor's loop — read (Subscription usage reader: OAuth /api/oauth/usage poll with degradation chain #31/API-mode usage reader from response headers (built, inactive) #32) → evaluate threshold (Threshold evaluation and date-sensitive Agent SDK credit meter handling #33) → step cap (Concurrency cap step-down as usage lever (3 to 2 to 1 to pause) #35) / apply modifiers (Override switch and peak-hour throttle policy modifiers #36) → pause + resume (Hard pause and two-tier automatic resume (reset-known sleep + backoff probe) #34) — is wired into the wave-loop driver (Wave-loop driver: end-to-end orchestration of the deterministic wave loop #29) so a real wave actually steps the cap and pauses/resumes on usage.
• The governing meter and current state (waiting-for-usage, override on/off, cap level) are exposed for the UI to read (UI itself is Phase 6); no hardcoded values.
• Every external call (OAuth endpoint, probe, SDK) retains an explicit success and failure path end-to-end; a usage-read failure degrades the run, never crashes it.
• An end-to-end test drives a simulated wave through threshold approach → cap step-down → pause → reset → resume from the per-issue marker, using a fake UsageReader.

Depends on: #29
Depends on: #33
Depends on: #34
Depends on: #35
Depends on: #36