feat(workflow): add optional precheck gate to skip empty runs

[Bartok9]

Refs #2297

Problem

Schedule-triggered workflows fire on a cron/interval. Today the only convergence control is per-step if: conditions. A poller workflow ("every 15m, check X, and if there's new work have an agent summarize it") still spins up the step chain — and any agent/webhook/message action — on every tick even when there's nothing to do. On a busy relay with many scheduled agent workflows that's a real, silent cost and a source of empty-run channel noise.

Why it matters

Buzz makes agents first-class; scheduled agent workflows are a natural pattern. A cheap gate that decides whether there's work before doing expensive work makes them affordable and quiet. This is the same zero-cost-gate idea we contributed to the OpenClaw (job.precheck) and Hermes (NO_WORK wake gate) cron systems.

What this adds

A workflow-level precheck evalexpr gate, evaluated once at run start, before any step:

name: pr-triage
trigger:
  on: schedule
  cron: "*/15 * * * *"
precheck: "trigger_open_pr_count > 0"   # same vars as step if:
steps:
  - id: summarize
    action: { send_message: { text: "…" } }

• precheck false → run short-circuits with zero step actions, returns Completed with a skipped-precheck trace entry.
• Absent precheck → unchanged behavior (serde default None).

Design / safety

• Additive. Existing workflows are byte-for-byte unaffected.
• Reuses evaluate_condition — inherits the same 4 KB expression cap + 100 ms timeout envelope as step if:. No new execution surface, no shell.
• Fail-open. A precheck that errors runs the steps (a bad gate never silently disables a workflow) and logs a warning — surfaced in the trace.
• Stateless in v1 (trigger context only), matching step if:.

How to test

cargo test -p buzz-workflow precheck   # parse + JSON round-trip + default-None
cargo clippy -p buzz-workflow          # clean
cargo fmt -p buzz-workflow --check

The gate's decision uses the same evaluate_condition primitive that's already covered by the executor condition tests (condition_*).

Notes / open to maintainer direction

• I chose a workflow-level field over "just gate step 1 with if:" for clean single-skip semantics + a place to hang a skipped metric later.
• Happy to restrict it to Schedule triggers only, or add a dedicated skipped run-status/metric, if preferred (see issue [Proposal] Scheduled-workflow precheck gate: skip runs with zero token/action cost #2297 comment).

Signed-off-by DCO.