Multi-account Phase 3 PR D: ClaudeProvider conformance to MultiAccountProvider

[hanrw]

Context

Phase 3 follow-up from #160 — Concrete PR D (optional, self-contained).

Scope

• This one touches probe internals so may prefer to handle in-house
• Pattern: activeProbe selection keyed by account ID instead of probe mode, per-account snapshots stored in a dictionary, switchAccount changes the active probe
• Each account maps to a CLI profile (claude --profile work) or API token env var via probeConfig

Notes

The existing ClaudeProvider already has dual probe support (CLI + API). Multi-account extends this to N probes, one per account. The activeProbe selection logic already exists — it just needs to be keyed by account ID instead of probe mode.

Dependencies

Depends on PR A (#164) and PR B for full functionality.

Ref: #160 (Phase 3 discussion in PR comments)