Add public skills + improve repo skill discovery and error UX (#8098)

1. Adds SkillScope::Public end-to-end (core + protocol) and loads skills
from the public cache directory
2. Improves repo skill discovery by searching upward for the nearest
.codex/skills within a git repo
3. Deduplicates skills by name with deterministic ordering to avoid
duplicates across sources
4. Fixes garbled “Skill errors” overlay rendering by preventing pending
history lines from being injected during the modal
5. Updates the project docs “Skills” intro wording to avoid hardcoded
paths

codex-rs/core/src/project_doc.rs

@@ -522,7 +522,7 @@ mod tests {
         let expected_path_str = expected_path.to_string_lossy().replace('\\', "/");
         let usage_rules = "- Discovery: Available skills are listed in project docs and may also appear in a runtime \"## Skills\" section (name + description + file path). These are the sources of truth; skill bodies live on disk at the listed paths.\n- Trigger rules: If the user names a skill (with `$SkillName` or plain text) OR the task clearly matches a skill's description, you must use that skill for that turn. Multiple mentions mean use them all. Do not carry skills across turns unless re-mentioned.\n- Missing/blocked: If a named skill isn't in the list or the path can't be read, say so briefly and continue with the best fallback.\n- How to use a skill (progressive disclosure):\n  1) After deciding to use a skill, open its `SKILL.md`. Read only enough to follow the workflow.\n  2) If `SKILL.md` points to extra folders such as `references/`, load only the specific files needed for the request; don't bulk-load everything.\n  3) If `scripts/` exist, prefer running or patching them instead of retyping large code blocks.\n  4) If `assets/` or templates exist, reuse them instead of recreating from scratch.\n- Description as trigger: The YAML `description` in `SKILL.md` is the primary trigger signal; rely on it to decide applicability. If unsure, ask a brief clarification before proceeding.\n- Coordination and sequencing:\n  - If multiple skills apply, choose the minimal set that covers the request and state the order you'll use them.\n  - Announce which skill(s) you're using and why (one short line). If you skip an obvious skill, say why.\n- Context hygiene:\n  - Keep context small: summarize long sections instead of pasting them; only load extra files when needed.\n  - Avoid deeply nested references; prefer one-hop files explicitly linked from `SKILL.md`.\n  - When variants exist (frameworks, providers, domains), pick only the relevant reference file(s) and note that choice.\n- Safety and fallback: If a skill can't be applied cleanly (missing files, unclear instructions), state the issue, pick the next-best approach, and continue.";
         let expected = format!(
-            "base doc\n\n## Skills\nThese skills are discovered at startup from ~/.codex/skills; each entry shows name, description, and file path so you can open the source for full instructions. Content is not inlined to keep context lean.\n- pdf-processing: extract from pdfs (file: {expected_path_str})\n{usage_rules}"
+            "base doc\n\n## Skills\nThese skills are discovered at startup from multiple local sources. Each entry includes a name, description, and file path so you can open the source for full instructions.\n- pdf-processing: extract from pdfs (file: {expected_path_str})\n{usage_rules}"
         );
         assert_eq!(res, expected);
     }
@@ -546,7 +546,7 @@ mod tests {
         let expected_path_str = expected_path.to_string_lossy().replace('\\', "/");
         let usage_rules = "- Discovery: Available skills are listed in project docs and may also appear in a runtime \"## Skills\" section (name + description + file path). These are the sources of truth; skill bodies live on disk at the listed paths.\n- Trigger rules: If the user names a skill (with `$SkillName` or plain text) OR the task clearly matches a skill's description, you must use that skill for that turn. Multiple mentions mean use them all. Do not carry skills across turns unless re-mentioned.\n- Missing/blocked: If a named skill isn't in the list or the path can't be read, say so briefly and continue with the best fallback.\n- How to use a skill (progressive disclosure):\n  1) After deciding to use a skill, open its `SKILL.md`. Read only enough to follow the workflow.\n  2) If `SKILL.md` points to extra folders such as `references/`, load only the specific files needed for the request; don't bulk-load everything.\n  3) If `scripts/` exist, prefer running or patching them instead of retyping large code blocks.\n  4) If `assets/` or templates exist, reuse them instead of recreating from scratch.\n- Description as trigger: The YAML `description` in `SKILL.md` is the primary trigger signal; rely on it to decide applicability. If unsure, ask a brief clarification before proceeding.\n- Coordination and sequencing:\n  - If multiple skills apply, choose the minimal set that covers the request and state the order you'll use them.\n  - Announce which skill(s) you're using and why (one short line). If you skip an obvious skill, say why.\n- Context hygiene:\n  - Keep context small: summarize long sections instead of pasting them; only load extra files when needed.\n  - Avoid deeply nested references; prefer one-hop files explicitly linked from `SKILL.md`.\n  - When variants exist (frameworks, providers, domains), pick only the relevant reference file(s) and note that choice.\n- Safety and fallback: If a skill can't be applied cleanly (missing files, unclear instructions), state the issue, pick the next-best approach, and continue.";
         let expected = format!(
-            "## Skills\nThese skills are discovered at startup from ~/.codex/skills; each entry shows name, description, and file path so you can open the source for full instructions. Content is not inlined to keep context lean.\n- linting: run clippy (file: {expected_path_str})\n{usage_rules}"
+            "## Skills\nThese skills are discovered at startup from multiple local sources. Each entry includes a name, description, and file path so you can open the source for full instructions.\n- linting: run clippy (file: {expected_path_str})\n{usage_rules}"
         );
         assert_eq!(res, expected);
     }