feat(installer): stop auto-indexing on install + ship opt-in front-load prompt hook

`codegraph install` no longer indexes the current directory — it wires up agents
only, and building a project's graph is always the explicit `codegraph init` /
`index`. Removes the global-vs-local inconsistency (a local install silently
indexed, a global one didn't) and the docs/behavior mismatch (#826). README
updated to match; the stale `init --index` note (indexing is default now) fixed.

Adds an opt-in Claude Code front-load hook: a `UserPromptSubmit` hook that runs
the new hidden `codegraph prompt-hook`, which injects codegraph_explore context
for structural ("how / where / trace / impact") prompts so the agent answers
from the graph instead of grepping to rebuild it. Prompted at install
(default-yes; Claude-only — the only agent with prompt hooks), removed on
uninstall, and `codegraph upgrade` self-heals it onto an already-configured
global Claude install. Strictly additive + degradable: non-structural prompts,
un-indexed projects, and any failure are silent no-ops. Disable without
uninstalling via CODEGRAPH_NO_PROMPT_HOOK=1.

7 new installer-targets contract tests (write / idempotent / opt-out round-trip /
sibling-preserved / uninstall / legacy-independent). Full suite green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -76,7 +76,7 @@ In a **new terminal**, run the installer to connect CodeGraph to the agents you
 codegraph install
 ```
 
-<sub>Detects and auto-configures Claude Code, Cursor, Codex CLI, opencode, Hermes Agent, Gemini CLI, Antigravity IDE, and Kiro — wiring the CodeGraph MCP server into each. **This is the step that connects CodeGraph to your agent;** installing the CLI in step 1 does not do it on its own. (Shortcut: `npx @colbymchenry/codegraph` downloads and runs this in one go.)</sub>
+<sub>Detects and auto-configures Claude Code, Cursor, Codex CLI, opencode, Hermes Agent, Gemini CLI, Antigravity IDE, and Kiro — wiring the CodeGraph MCP server into each. **This is the step that connects CodeGraph to your agent;** installing the CLI in step 1 does not do it on its own. It only wires up your agent — it does **not** index any code; building each project's graph is the separate `codegraph init` in step 3. (Shortcut: `npx @colbymchenry/codegraph` downloads and runs this in one go.)</sub>
 
 ### 3. Initialize each project
 
@@ -341,7 +341,8 @@ The installer will:
 - Ask whether configs apply to all your projects or just this one
 - Write each chosen agent's MCP server config, plus a small marker-fenced CodeGraph section in the agent's instructions file (`CLAUDE.md` / `AGENTS.md` / `GEMINI.md`) — that's how subagents and non-MCP agents learn the `codegraph explore` command, since the MCP server's own guidance only reaches the main agent. Removed cleanly by `codegraph uninstall`.
 - Set up auto-allow permissions when Claude Code is one of the targets
-- Initialize your current project (local installs only)
+
+The installer **wires up your agents only — it does not index your code.** After it finishes, build each project's graph yourself with `codegraph init` (step 3). One global `codegraph install` covers every project; you run `codegraph init` once per project.
 
 **Non-interactive (scripting / CI):**
 
@@ -466,7 +467,7 @@ The exact text is `src/mcp/server-instructions.ts` — the single source of trut
 codegraph                         # Run interactive installer
 codegraph install                 # Run installer (explicit)
 codegraph uninstall               # Remove CodeGraph from your agents (inverse of install)
-codegraph init [path]             # Initialize in a project (--index to also index)
+codegraph init [path]             # Initialize a project + build its graph (one step)
 codegraph uninit [path]           # Remove CodeGraph from a project (--force to skip prompt)
 codegraph index [path]            # Full index (--force to re-index, --quiet for less output)
 codegraph sync [path]             # Incremental update