ACP: expose contextTier as a session config option (parity with interactive /model picker)

Describe the feature or problem you'd like to solve

The interactive CLI lets you change the context window tier mid-session (via the /model context tier picker), but the ACP server does not expose contextTier as a session config option. ACP clients can only set the tier at spawn time with --context <tier>, and have no way to discover or change it once the session is running.

Proposed solution

Expose the context tier over ACP with the same shape already used for reasoning_effort:

1. Include a contextTier entry in the configOptions array returned by session/new (and session/load), of type: "select", with the eligible values for the current model (default, long_context) and the current value.
2. Accept session/set_config_option with configId: "contextTier" and apply it to the live session, exactly as the /model picker does interactively.
3. Only advertise the option for tiered-pricing models, mirroring the interactive behaviour ("context tier picker appears for eligible models").

This closes a parity gap rather than adding new functionality: the capability already exists in the runtime and is reachable from the interactive UI and from settings.json; it is just not reachable through ACP.

Issue #1994 ("Allow reasoning effort discovery and setting via ACP") is the direct precedent — reasoning_effort is now a first-class ACP config option, and contextTier is the remaining sibling setting that isn't.

Example prompts or workflows

1. An ACP client (Zed, or any ACP editor integration) shows a "Context Size" picker next to the model and reasoning-effort pickers, populated from session/new, instead of having to hardcode the tier list per CLI version.
2. A user on Claude Opus hits a large refactor mid-conversation and switches from default to long_context without losing session state — today the client must kill and respawn copilot --acp --context long_context and resume, or the control is simply hidden.
3. A client drops back from long_context to default after the large task is done, to avoid the surcharge for the rest of the conversation.
4. A client reads the current tier from session/load when reconnecting to an existing session, so the UI reflects reality instead of guessing.

Additional context

Reproduced against GitHub Copilot CLI 1.0.75.

The interactive CLI supports hot changes — from copilot help config:

contextTier: context window tier for tiered-pricing models (e.g., "default" or "long_context").
  - Can also be set with --context flag (overrides persisted setting)
  - Can be changed with /model command (context tier picker appears for eligible models)

ACP does not. Minimal probe over copilot --acp (initialize → session/new → session/set_config_option):

configOptions ids: ["mode","model","reasoning_effort","allow_all"]
contextTier -> {"code":-32602,"message":"Unknown config option 'contextTier'."}
context     -> {"code":-32602,"message":"Unknown config option 'context'."}
contextSize -> {"code":-32602,"message":"Unknown config option 'contextSize'."}

So reasoning_effort is exposed but contextTier is not, even though both are per-session model settings adjustable from the interactive /model flow.

Current workaround and why it's poor. The tier can only be pinned at spawn time via --context <tier>. Changing it for a running conversation requires terminating the agent process and respawning it with a different flag, then reloading the session — an intrusive lifecycle operation for what is a per-request setting, and one that ACP clients cannot make feel like the interactive picker.

Start with the existing reasoning_effort ACP config option as the precedent, then trace session/new and session/load advertisement and session/set_config_option handling. Compare those paths with the interactive /model picker and --context behavior. Done means eligible models expose the current contextTier and clients can change it during a live session without restarting.