feat: add Codex SessionStart reminder hook for codebase-memory tool discovery

[dergachoff]

Problem

codebase-memory-mcp install already configures Codex CLI with MCP config and .codex/AGENTS.md, but it does not install a Codex hook reminder. The README currently lists Codex hooks as -.

That leaves Codex users exposed to a known deferred-tool discovery failure mode: a broad tool_search query can omit an expected MCP tool from the first result set, even when the query includes the tool name. In our local Codex setup, a broad codebase-memory discovery query omitted search_graph; a second exact-name tool_search for search_graph found it.

This is tracked upstream in Codex as openai/codex#21503.

Proposed Solution

Add optional Codex CLI SessionStart hook installation alongside the existing Codex MCP config and .codex/AGENTS.md installation.

The hook should be non-blocking and only print a compact discovery protocol. It should not install a PreToolUse gate for Codex yet.

Suggested reminder text:

Code discovery protocol:
1. For source-code exploration, use codebase-memory-mcp first.
2. If MCP tools are not callable, run tool_search for:
   "search_graph trace_path get_code_snippet query_graph search_code index_repository codebase-memory-mcp"
3. If an expected tool is still missing, retry tool_search with the exact tool name, for example "search_graph"; absence from the first broad result is not proof the tool is unavailable.
4. Establish the project with list_projects/index_status; run index_repository if missing or stale.
5. Use search_graph for symbols, search_code for fuzzy text, get_code_snippet for exact qualified names, trace_path for callers/callees, and query_graph for complex graph queries.
6. Fall back to shell search/read only for docs/configs/non-code files or after MCP discovery genuinely fails.

Why SessionStart, not PreToolUse?

Claude Code currently gets a PreToolUse gate, but that gate has open UX issues around over-blocking and session detection. Codex also has different hook semantics and still has upstream limitations around full tool coverage / input rewrite.

A Codex SessionStart reminder is lower-risk:

• no blocked reads or grep calls
• works across startup/resume/clear lifecycle events
• preserves user autonomy
• directly addresses the deferred tool_search exact-name retry workaround

Acceptance Criteria

• codebase-memory-mcp install detects Codex and can install/update a Codex SessionStart hook.
• The hook writes to Codex's hook config without disturbing existing user hooks.
• uninstall removes only the codebase-memory-managed Codex hook.
• README Multi-Agent Support table changes Codex hooks from - to SessionStart reminder.
• Tests cover fresh install, preserving existing hooks, replacing old managed hook text, and uninstall.

I'm happy to make an upstream PR if maintainers agree with this direction.