[codex] narrow unused skills intro export (#29991)

## Summary

- stop publicly re-exporting the internally used
`SKILLS_INTRO_WITH_ALIASES` constant
- keep the constant and all skills rendering behavior unchanged
- preserve every integration helper, API, fixture, assertion, and module
used by tests

## Scope guardrails

This revision keeps all remote/network-facing functionality and every
line introduced by `jif <jif@openai.com>`.

Following the test-preservation audit, it also restores the in-process
RMCP test transport, the original `codex-mcp` fixture,
`PluginLoadOutcome::effective_skill_roots` and its assertions, the
`EffectiveSkillRoots` API family, the test-only apps renderer, and the
TUI dead-code annotation. Those files now match the PR base exactly.

No test imports or directly references the remaining public skills
export being narrowed.

## Validation

- repository-wide test-reference audit: no test-used code remains
deleted or narrowed
- deleted-line `git blame` audit: zero Jif-authored deletions
- `cargo test -p codex-core-plugins -p codex-mcp -p codex-rmcp-client
--lib`: 467 passed
- `cargo test -p codex-core --lib apps::render`: 2 passed
- `cargo test -p codex-core-skills --lib render::tests`: 19 passed
- `cargo check -p codex-core-skills --all-targets`: passed
- `just fix -p codex-core-skills`: passed
- `just fmt`: passed
- `git diff --check`: passed

The full local `codex-core-skills` suite passed 106/108 tests; two
loader tests detected an ambient repository skills root outside the
package and failed their isolation assertions. The scoped renderer suite
and all-target compile pass, and CI runs in an isolated environment.

Final code delta: 1 insertion, 2 deletions across 2 files.

codex-rs/core-skills/src/lib.rs

@@ -24,7 +24,6 @@ pub use render::AvailableSkills;
 pub use render::SKILLS_HOW_TO_USE_WITH_ABSOLUTE_PATHS;
 pub use render::SKILLS_HOW_TO_USE_WITH_ALIASES;
 pub use render::SKILLS_INTRO_WITH_ABSOLUTE_PATHS;
-pub use render::SKILLS_INTRO_WITH_ALIASES;
 pub use render::SkillMetadataBudget;
 pub use render::SkillRenderReport;
 pub use render::build_available_skills;

codex-rs/core-skills/src/render.rs

@@ -26,7 +26,7 @@ pub const SKILL_DESCRIPTION_TRUNCATED_WARNING_WITH_PERCENT: &str = "Skill descri
 pub const SKILL_DESCRIPTIONS_REMOVED_WARNING_PREFIX: &str =
     "Exceeded skills context budget. All skill descriptions were removed and";
 pub const SKILLS_INTRO_WITH_ABSOLUTE_PATHS: &str = "A skill is a set of instructions provided through a `SKILL.md` source. Below is the list of skills that can be used. Each entry includes a name, description, and source locator. `file` locators are on the host filesystem, `environment resource` locators are owned by an execution environment, `orchestrator resource` locators are opaque non-filesystem resources, and `custom resource` locators use their provider's access mechanism.";
-pub const SKILLS_INTRO_WITH_ALIASES: &str = "A skill is a set of local instructions to follow that is stored in a `SKILL.md` file. Below is the list of skills that can be used. Each entry includes a name, description, and a short path that can be expanded into an absolute path using the skill roots table.";
+const SKILLS_INTRO_WITH_ALIASES: &str = "A skill is a set of local instructions to follow that is stored in a `SKILL.md` file. Below is the list of skills that can be used. Each entry includes a name, description, and a short path that can be expanded into an absolute path using the skill roots table.";
 pub const SKILLS_HOW_TO_USE_WITH_ABSOLUTE_PATHS: &str = r###"- Discovery: The list above is the skills available in this session (name + description + source locator). `file` entries live on the host filesystem, `environment resource` entries are owned by their execution environment, `orchestrator resource` entries must be accessed through `skills.list` and `skills.read`, and `custom resource` entries use their provider's access mechanism.
 - Trigger rules: If the user names a skill (with `$SkillName` or plain text) OR the task clearly matches a skill's description shown above, you must use that skill for that turn. Multiple mentions mean use them all. Do not carry skills across turns unless re-mentioned.
 - Missing/blocked: If a named skill isn't in the list or its source can't be read, say so briefly and continue with the best fallback.