Add thread metadata

[evan-oai]

Adds an optional execution_environment value to V2 turn/start and turn/steer requests so clients that know they are running on a remote host can mark the turn as remote. The app-server analytics reducer carries that value into codex_turn_event, which lets downstream analytics count remote turns and remote-thread activity without adding hostnames, paths, URLs, or other environment-derived identifiers.

The request field is experimental, and the public stable request schema remains filtered; the generated fixtures only pick up the shared enum definition.

Validation:

• cargo test -p codex-analytics
• cargo test -p codex-app-server-protocol
• cargo check -p codex-app-server -p codex-exec -p codex-tui -p codex-app-server-test-client
• cargo fmt --check

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