feat(api): thread context.Context through the client; cancel cleanly on SIGINT

[kush2shah]

Fixes #25. Fixes #34.

What this does

One PR for both issues because #34's complete fix requires #25's plumbing — a signal handler without request contexts only cancels between polls, not during one.

#34 — signal handling. Execute() now runs the command tree under signal.NotifyContext(SIGINT, SIGTERM) via rootCmd.ExecuteContext. The first Ctrl-C cancels cmd.Context() everywhere: the in-flight HTTP request aborts, --wait loops exit through their cancellation branches, and tendlc's awaitTerminal emits the receipt with bandwidthId before the process exits — the accepted-but-unprinted brand ID from the issue's reproduction is no longer lost. A second Ctrl-C is untrapped (NotifyContext stops after one signal) and hard-exits the Go-default way.

#25 — context threading. Requester and every api.Client method take ctx context.Context as the first parameter, and requests are built with http.NewRequestWithContext. On the open design question, this changes the signatures in place rather than adding parallel …Ctx methods, as the issue leans: one mechanical diff, no lingering dual API, pre-1.0. All ~150 call sites migrated: commands pass cmd.Context(); the internal/sip, internal/tendlc, and internal/customerprofile service layers thread ctx through their methods; --wait helper functions take ctx; the testutil.FakeClient and test fakes match the new interface.

Poll wiring. Every cmdutil.Poll site (13 of them) now sets PollConfig.Context, so the Context field added in the 10DLC foundation work finally has a producer.

Scope note

auth.TokenManager.GetToken (token exchange) still runs without a context — it has its own timeout, and that path is entangled with the #24 auth-status work, so it's deliberately left out.

Tests

• TestClient_ContextCancelsInFlightRequest — a context deadline aborts an in-flight HTTP request against a deliberately-hanging server (fails against the old http.NewRequest build).
• TestExecuteCancelsCommandContextOnSIGINT — sends a real SIGINT to the test process and asserts the command's context cancels and Execute returns (//go:build !windows). The issue's "second Ctrl-C hard-exits" behavior comes from NotifyContext semantics and can't be asserted in-process.
• Existing suite: go build, go vet, gofmt clean; full tests pass; internal/api, internal/cmdutil, and cmd race-clean over -count=3.

Docs

AGENTS.md's two interim notes — the untrappable-Ctrl-C caveat on the receipt guarantee (brands and campaigns) — are replaced with the new guarantee: a single Ctrl-C emits the receipt; only a second one kills the process without output.

[bwappsec]

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