Don't add a separate fbuild-broker crate — keep fbuild close to a monocrate

[zackees]

Position

fbuild should stay as close to a monocrate as possible. We do not want a separate fbuild-broker crate at all.

PR #559 (feat/v1-broker-adoption) introduces a new crates/fbuild-broker/ crate (protocol + service + session modules) to adopt the running-process v1 broker session. That takes the workspace from 11 crates to 12 in the wrong direction.

Why fold it in instead

• The broker session is consumed by exactly one crate — fbuild-daemon (it is the tokio process that calls …BrokerSession::adopt). A standalone crate adds a public API surface, a Cargo.toml, a README, and a dependency edge for code with a single in-tree consumer.
• The "protocol/service/session" split is internal organization, which a module tree expresses just as well as a crate — without the packaging overhead.
• Keeping it in-crate matches the existing convention: per-platform orchestrators live as modules under fbuild-build, not as one crate each.

Ask

Move the running-process v1 adoption out of a standalone crate and into an existing one — fbuild-daemon is the natural home:

crates/fbuild-daemon/src/broker/
    mod.rs        (was fbuild-broker/src/lib.rs)
    protocol.rs
    service.rs
    session.rs

If any of it is genuinely needed by another crate (e.g. the CLI's daemon running-process diagnostic prints service::SERVICE_NAME / MIN_VERSION), expose just those constants from wherever they land rather than keeping a whole crate alive for them — or have the CLI ask the daemon over the existing HTTP API.

Acceptance

• No crates/fbuild-broker/ directory; workspace stays at 11 crates.
• running-process v1 adoption (protocol/service/session) lives as a module inside an existing crate (fbuild-daemon preferred).
• register_payload_protocol! pin and the RUNNING_PROCESS_DISABLE=1 rollback behavior are preserved exactly.
• fbuild daemon running-process diagnostic still prints the same facts.
• cargo test/clippy green.

Context

• PR feat(broker): adopt running-process v1 broker session #559 — feat(broker): adopt running-process v1 broker session
• Related: running-process broker: fbuild adoption tracker #510, v1 broker adoption: fbuild — full broker migration (inventory + ServiceDefinition + CacheManifest) zackees/running-process#437