[codex] Add CODEX_HOME instructions contributor

[anp-oai]

Why

CODEX_HOME instruction discovery is currently implemented inside codex-core, even though it is a host-level source of global instructions. Moving that behavior into a dedicated crate keeps core focused on rendering and lifecycle semantics and creates a natural home for future CODEX_HOME-specific behavior.

What changed

• Adds the codex-home crate.
• Adds CodexHomeInstructionsContributor, constructed from a CODEX_HOME path without depending on codex-core.
• Preserves AGENTS.override.md precedence and fallback to AGENTS.md.
• Preserves empty-file handling, lossy UTF-8 warnings, read failures, and absolute source reporting.
• Allows global instructions to load without an execution environment.
• Reexports the contributor from codex-core-api.

This PR defines the contributor but does not register or resolve it in production hosts yet.

Stack

1. [codex] Characterize global instruction lifecycle #26830: lifecycle characterization tests
2. [codex] Add global instructions contributor API #26831: extension API primitives
3. [codex] Add CODEX_HOME instructions contributor #26832: CODEX_HOME contributor crate
4. [codex] Persist structured instruction snapshots #26833: structured snapshot and history machinery
5. [codex] Adopt global instructions contributors #26834: contributor adoption and compatibility removal

Test plan

• just test -p codex-home
• Coverage includes override precedence, fallback, whitespace-only and missing files, lossy UTF-8 warnings, source paths, read failures, and loading without an environment.
• just bazel-lock-check

[anp-oai]

this should have a doc comment