When using OpenClaude with GPT-5/Codex backends, slash commands such as
/skill-name trigger a Skill tool call without the required `skill` parameter,
resulting in: "The required parameter `skill` is missing".

Root cause: OpenAI function calling relies heavily on schema descriptions to
infer parameter values. The previous description did not make the
slash-to-parameter mapping explicit, causing GPT-5 to call Skill({}) or
Skill({skill: undefined}) instead of Skill({skill: "skill-name"}).

Changes:
- inputSchema: expand `skill` description to explicitly document that the
  leading slash must be stripped (/commit → skill: "commit") with concrete
  input/output examples for GPT-style function calling
- validateInput: add a null/undefined guard before skill.trim() to produce
  an actionable error message when non-Claude runtimes omit the parameter
- prompt.ts: append a short IMPORTANT block for OpenAI/GPT function calling
  that reinforces the required parameter contract and slash-stripping rule

Affected provider: OpenAI-compatible / Codex backends only.
Claude (Anthropic) behaviour is unchanged.

… calling

Extend the GPT-5 compatibility fix from SkillTool to all other tools.
GPT-5 function calling relies heavily on description fields to decide
whether to pass a parameter. Without explicit REQUIRED markers and
concrete examples, GPT-5 omits required string parameters, causing
InputValidationError across Agent, Bash, FileRead, FileEdit, Glob,
Grep, WebFetch, WebSearch, and FileWrite tools.

Changes per tool:
- AgentTool: description + prompt → REQUIRED with examples
- BashTool: command → REQUIRED with examples
- FileReadTool: file_path → REQUIRED with absolute path example
- FileEditTool/types.ts: file_path, old_string, new_string → REQUIRED
- GlobTool: pattern → REQUIRED with glob examples
- GrepTool: pattern → REQUIRED with regex examples
- WebFetchTool: url + prompt → REQUIRED with examples
- WebSearchTool: query → REQUIRED with examples
- FileWriteTool: file_path + content → REQUIRED (already committed)