fix(installer): instructions block is scope-neutral — conditional on .codegraph/ existing (#820)

A global install writes the block into user-scope files
(~/.claude/CLAUDE.md, ~/.codex/AGENTS.md) that apply to EVERY repo the
user opens — the unconditional "This repository is indexed" claim was
false in unindexed ones and would send subagents into failing codegraph
calls, the exact noise the unindexed-session policy (#817) eliminates.
Now: "In repositories indexed by CodeGraph (a .codegraph/ directory
exists) …" plus an explicit skip-entirely line for the no-index case.

Residual: the delegation A/B validated the assertive project-scoped
wording; the conditional form keeps the same active ingredients (the
codegraph name + both command surfaces in the relay-able slot) — fold a
re-check into the next delegation A/B run.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

src/installer/instructions-template.ts

@@ -29,12 +29,23 @@
 export const CODEGRAPH_SECTION_START = '<!-- CODEGRAPH_START -->';
 export const CODEGRAPH_SECTION_END = '<!-- CODEGRAPH_END -->';
 
-/** The full block, markers included, exactly as written to disk. */
+/**
+ * The full block, markers included, exactly as written to disk.
+ *
+ * The wording is deliberately CONDITIONAL ("in repositories indexed by…"):
+ * a global install writes this into a user-scope file (~/.claude/CLAUDE.md,
+ * ~/.codex/AGENTS.md) that applies to every project the user opens —
+ * including unindexed ones, where an unconditional "this repository is
+ * indexed" claim would send subagents into failing codegraph calls (the
+ * noise the unindexed-session policy exists to prevent).
+ */
 export const CODEGRAPH_INSTRUCTIONS_BLOCK = `${CODEGRAPH_SECTION_START}
 ## CodeGraph
 
-This repository is indexed by CodeGraph — a pre-built code knowledge graph. Reach for it BEFORE grep/find or reading files when you need to understand or locate code:
+In repositories indexed by CodeGraph (a \`.codegraph/\` directory exists at the repo root), reach for it BEFORE grep/find or reading files when you need to understand or locate code:
 
 - **MCP tools** (when available): \`codegraph_explore\` answers most code questions in one call — the relevant symbols' verbatim source plus the call paths between them. \`codegraph_node\` returns one symbol's source + callers, or reads a whole file with line numbers. If the tools are listed but deferred, load them by name via tool search.
 - **Shell** (always works): \`codegraph explore "<symbol names or question>"\` and \`codegraph node <symbol-or-file>\` print the same output.
+
+If there is no \`.codegraph/\` directory, skip CodeGraph entirely — indexing is the user's decision.
 ${CODEGRAPH_SECTION_END}`;