[codex] Load user instructions through an injected provider

[anp-oai]

Why

We want to remove implicit use of $CODEX_HOME from codex-core and make embedders responsible for supplying user-level instructions. This also ensures user instructions load when no primary environment is selected.

What changed

Stacked on #27415, which makes codex exec surface thread-scoped runtime warnings.

• Added UserInstructionsProvider to codex-extension-api, with absolute source attribution and recoverable loading warnings.
• Added codex-home with the filesystem-backed provider for AGENTS.override.md and AGENTS.md, preserving precedence, fallback, trimming, lossy UTF-8 handling, and the existing uncapped global instruction size.
• Removed global instruction loading from Config and require ThreadManager callers to inject a provider.
• Load provider instructions once for each fresh root runtime, including runtimes without a primary environment. Running sessions retain their snapshot, while child agents inherit the parent snapshot without invoking the provider.
• Keep provider instructions separate while loading project AGENTS.md, then assemble the model-visible instructions with the existing ordering, source attribution, warning, and turn-context behavior.
• Wired the Codex home provider through the CLI, app server, MCP server, core facade, and thread-manager sample.

Validation

• just test -p codex-home -p codex-extension-api
• just test -p codex-core agents_md
• just test -p codex-core guardian
• just test -p codex-app-server thread_start_without_selected_environment_includes_only_global_instruction_source
• just test -p codex-exec warning
• just bazel-lock-check

[chatgpt-codex-connector]

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