fix(mcp): use absolute path for Codex setup

[apanasenko-oai]

Why

Codex Desktop may not inherit the interactive shell's PATH. The installers currently recommend:

codex mcp add fff -- fff-mcp

This can leave fff configured but unavailable because Codex cannot resolve fff-mcp.

What changed

• Update the macOS/Linux and Windows installers to register the installed binary using its quoted absolute path.
• Add Codex setup examples for Homebrew and the one-line installer.
• Document the resulting ~/.codex/config.toml entry and restart requirement.

Verification

Confirmed that an installation path containing spaces produces:

codex mcp add fff -- "/private/tmp/fff path/bin/fff-mcp"

Also verified the Homebrew command creates:

[mcp_servers.fff]
command = "/opt/homebrew/bin/fff-mcp"

and successfully initializes the MCP server and performs a repository grep.