Pluggable ConfigSource: FileSource (default) + RemoteSource + --config-source

[rshade]

Context

The control plane stores per-tenant fleet config and serves it over an API. The
OSS CLI must be able to pull its config from a remote control plane (e.g.
api.agentic-fleet.ai) or read a local file — without the engine depending on
the control plane. This is the key binding point (open client, swappable
control plane).

Problem

Config loading is currently hard-wired to local file probing in
internal/fleet/load.go (LoadConfig/probeConfigPath). There is no seam to
load config from anywhere else, and no way for a hosted control plane to be the
source of truth.

Proposed approach

Introduce a pluggable source in the public pkg/ (depends on the pkg/fleet
extraction):

ConfigSource interface { Load(ctx) (*Config, error) }
  ├─ FileSource    // wraps today's local probe/load — the DEFAULT
  └─ RemoteSource  // GET <url> (+ bearer token) → fleet.SchemaVersion JSON

• FileSource is the default; the CLI stays fully standalone with no
  network, token, or SaaS required.
• RemoteSource: HTTP GET a configurable URL returning the fleet-config
  JSON contract; bearer token from env/config; optional ?version=N for
  reproducible pins; local cache with offline fallback (keep working when
  the control plane is unreachable — offline-tolerant cached-config behavior).
• CLI wiring: --config-source <url> flag + env var + config stanza.
  Default remains local file. Do not hardcode any domain — vendor-neutral;
  any conformant URL (incl. self-hosted) works.

Affected files

• internal/fleet/load.go (refactor loading behind the interface)
• cmd/root.go (persistent flag), relevant subcommands
• New pkg/…/configsource.go

Acceptance criteria

• ConfigSource interface + FileSource (default) + RemoteSource in
  pkg/.
• gh-aw-fleet list --config-source https://… loads config from a remote
  endpoint using a bearer token from env/config.
• No domain hardcoded anywhere; default behavior (local file) unchanged when
  no source is specified.
• RemoteSource caches the last good response and falls back to it offline.
• ?version=N honored for reproducible pulls.
• godoc on exported identifiers; make ci green.

Dependencies

• Depends on the public pkg/fleet extraction (foundational issue).
• Implements the client side of the wire contract (separate contract issue).

References

• agentic-fleet/control-plane/docs/architecture.md — "The binding point".