fix(elixir): keep ssh retries in orchestrator

[mstrautmann-oai]

Context

SSH failover currently happens inside AgentRunner, which can bypass per-host caps and rerun a ticket on a second host after the first host already started setup.

TL;DR

Keep each worker run on one SSH host and let the orchestrator own retries.

Summary

• Remove AgentRunner's internal cross-host failover loop
• Keep a worker lifetime pinned to one selected SSH host
• Add a regression test proving startup failure on one host does not fall through to another

Alternatives

• Keep internal failover and classify retryable errors, but that duplicates orchestrator scheduling logic
• Only patch per-host cap enforcement, but invisible cross-host reruns would still risk duplicate side effects

Test Plan

• make -C elixir all
• cd elixir && mise exec -- mix test test/symphony_elixir/core_test.exs:1167 test/symphony_elixir/core_test.exs:1237 test/symphony_elixir/core_test.exs:1337 --seed 0

[Copilot]

Pull request overview

This PR moves SSH retry/failover ownership out of AgentRunner and into the orchestrator by ensuring a single agent run stays pinned to one selected SSH host. This prevents a single worker lifetime from silently hopping to another host (which could bypass per-host caps and risk duplicate side effects).

Changes:

• Remove AgentRunner’s internal multi-host failover loop; it now runs on exactly one worker_host.
• Introduce selected_worker_host/2 to pick a single host (preferred host if provided, otherwise first configured host / local).
• Add a regression test asserting a startup failure on one host is surfaced and does not fall through to another host.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
elixir/lib/symphony_elixir/agent_runner.ex	Removes cross-host retry logic and pins each run to a single selected worker_host.
elixir/test/symphony_elixir/core_test.exs	Adds a regression test that verifies startup failures don’t trigger silent host hopping.

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