feat: declare per-model capabilities on session/new

[zxch3n]

Why

configOptions is rebuilt on every model switch: the Fast toggle is present
only while supportsFastMode, and the effort list belongs to the current model.
A client reading the session/new snapshot therefore learns nothing about any
other model — and ACP has no request that asks.

Lody was reading those options as an agent-wide capability catalog, so selecting
a model other than the probed one produced answers like "this agent has no fast
mode" for a model that does, or the reverse.

What

Attach the catalog the SDK already handed us to the session/new response:

{
  "sessionId": "…",
  "modes": { /* unchanged */ },
  "configOptions": [ /* unchanged */ ],
  "_meta": {
    "lody": {
      "modelCapabilities": {
        "version": 1,
        "models": {
          "sonnet": { "effortValues": ["low", "medium", "high"], "fastMode": true },
          "fable":  { "fastMode": false }
        }
      }
    }
  }
}

Built from the ModelInfo[] already in hand — supportedEffortLevels (only
when supportsEffort) and supportsFastMode. No new source of truth, no extra
request. effortValues is omitted rather than emptied for a model that does not
support effort, so "no effort control" and "an empty list" stay distinguishable.

Contract

Self-declared and advisory. It may describe a control the snapshot does not
carry; it never grants permission, authorizes a value, or rejects one. The live
session state remains the authority.

A model absent from models is unknown, not unsupported. The consumer
therefore reads the catalog whole or ignores it whole, and treats it as stale
after a TTL or a change of adapter version.

Emitted only when the model list is non-empty, so a client that ignores _meta
sees a byte-identical response.

Consumer

Read by Lody in LodyAI/Lody#333, which ships first and reads an absent
declaration as "nothing to say" — so this can merge on its own schedule.

Test

npx tsc --noEmit, npx eslint src/acp-agent.ts, and prettier --check clean.

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