Validate operator config files upfront; reject unknown YAML fields everywhere

[guettlibot]

Problem

Unknown fields in prompt front-matter (e.g. `processingLabel` instead of
`inProcessLabel`) are caught by the strict YAML decoder (`KnownFields(true)`)
and logged as errors — but only at run-time, once per cron tick. A typo in a
config file therefore causes every cron invocation to fail until someone
notices the log.

Real incident: `processingLabel` vs `inProcessLabel` in
`loop/sharedinbox/prompts/code.md` and `plan.md` went undetected across
many cron runs.

Requirements

1. Strict YAML everywhere — `KnownFields(true)` must be used for every
   config file parsed by agentloop: `acpx-config`, `config.yaml`, and all
   prompt `.md` front-matter. Prompt and acpx-config already do this; verify
   `config.yaml`.

2. Upfront validation on startup — before dispatching any agent work,
   `agentloop` and `agentloop cron` should load and validate all config
   files in the loop directory. Any unknown field or missing required field
   should abort the run with a clear, actionable error that names the file and
   field.

3. `agentloop validate` subcommand — a fast, side-effect-free command
   that checks every config file in a loop directory and prints errors, for
   use in CI / pre-commit hooks:

   agentloop validate [--loop <dir>]
   

   Exit 0 = all configs valid. Exit 1 = one or more errors, all listed.

Acceptance criteria

• `agentloop validate` with a prompt containing `processingLabel` exits 1
  and names the file and field.
• `agentloop` / `agentloop cron` with the same bad config exits non-zero
  before spawning any agent.
• All existing config structs use `KnownFields(true)`.