* feat: Amp support - read the local thread mirror (#1572)
Amp (Sourcegraph's coding agent) mirrors every thread it serves as one
JSON document per thread at ~/.local/share/amp/threads/ (AMP_DATA_DIR
honored, comma-separated). Each file is { id, messages,
usageLedger?{events} }; the ledger's events are the per-request record
(tokens.input/output, cache tokens joined from the billed message
through toMessageId -> messages[].messageId), with a per-assistant-
message usage fallback for threads without a ledger and a total-only
record billing as output.
Amp bills a subscription and records no charged dollars, so every call
is priced from the underlying model's token rates and marked
costIsEstimated; the recorded credits field is ignored (no published
per-credit dollar rate). Thread files carry no cwd, so sessions group
under the provider name. Registered eager; parse version
usage-ledger-v1; no artwork yet (mono-initial fallback, like Codebuff).
Surfaces: CLI registry + doctor probe roots, Electron sandbox read
paths + menubar catalog (Amp after Cursor, matching the Swift order),
Windows tray tab color + usage_guard root, GNOME prefs dropdown, macOS
ProviderConnectionCatalog, docs/providers/amp.md, README tool list,
CHANGELOG.
* fix: address Amp provider review feedback
