feat(agent): add qn agent context + agent discovery breadcrumbs (DX-5704)

[johnpmitsch]

What

Adds qn agent context — a new agent command group whose context subcommand prints a single, self-contained, version-stamped usage guide for AI agents — plus discovery breadcrumbs pointing to it.

Closes DX-5704 — https://linear.app/quicknode/issue/DX-5704/add-qn-agent-context-command-agent-discovery-breadcrumbs

Why

A coding agent dropped into qn has no single authoritative place to learn how to operate it correctly on the first try — auth resolution, the non-TTY output contract, exit-code semantics, confirmation gating, and retry/idempotency rules are scattered across --help and the SDK. This embeds one guide the agent can print offline, and points to it from where agents look.

What's in it

• qn agent context — prints the guide to stdout, exits 0 with no auth and no network. Joins the no-SDK dispatch group (auth/completions), so it never triggers the API-key path. Guide is embedded via include_str! from src/commands/agent/context.md; leads with control-flow-critical material (auth, output contract, exit codes, confirmation, retry/idempotency) before the command catalog and workflow recipes.
• Output contract — reads global.format directly (not resolve_format), so no flag → markdown (not the piped-default toon). -o json → { version, guide } envelope; -o yaml/-o toon/-o table → markdown + a one-line stderr notice (suppressed by --quiet). Workflow examples use the default format so piped agents get token-efficient toon; recipes model create-paused → show → activate and chain returned IDs.
• Breadcrumbs — qn --help footer line and a stderr tip after successful qn auth login, one canonical phrase reused in both.
• stream create --fix-block-reorgs help now documents the false (disabled) default.
• CLAUDE.md sync rule to keep context.md aligned with the command surface.

Out of scope (by decision)

Error-exit breadcrumbs (no changes to errors.rs/main.rs); -o json structured command catalog; error.hint envelope field; future qn agent install/mcp/doctor (namespace left open only).

Testing

• cargo build --release, cargo clippy --all-targets -- -D warnings, cargo fmt --check — all clean.
• cargo test — 229 pass.
• Manual: offline exit 0 with empty config; version interpolated ({{VERSION}} gone); -o json valid envelope; -o toon prints markdown + stderr notice; --quiet suppresses it; breadcrumbs present in qn --help and qn agent --help; the stream recipe's flags parse and reach the API.

No automated tests added: the in-process harness doesn't capture stdout, and the behavior is "print an embedded string."