feat(mcp): codegraph_node reads files like the Read tool — offset/limit, byte-parity (#738)

Makes codegraph_node a drop-in faster Read for indexed source files (file-read mode: <n>\t<line> like Read, offset/limit, + blast-radius header; symbolsOnly for the map). Fixes the old file-view dropping imports/line-numbers. #383/#527 preserved. Validated by A/B: explore/node already return source + line numbers, so Read=0 when used. Includes the A/B eval harness scripts. Full suite green (1270).

src/mcp/server-instructions.ts

@@ -48,7 +48,8 @@ typically one to a few calls; a grep/read exploration is dozens.
 - **"How does X reach/become Y? / the flow / the path from X to Y"** → \`codegraph_explore\`, naming the symbols that span the flow (e.g. \`mutateElement renderScene\`) — it surfaces the call path among them, including dynamic-dispatch hops (callbacks, React re-render, JSX children) grep can't follow
 - **"What is the symbol named X?" (just its location)** → \`codegraph_search\`
 - **"What calls this?" / "What does this call?" / "What would changing this break?"** → \`codegraph_callers\` / \`codegraph_callees\` / \`codegraph_impact\`
-- **About to read or edit a symbol you can name** → \`codegraph_node\` (SECONDARY — the after-explore depth tool) instead of \`Read\`: it returns the **verbatim current on-disk source** (safe to base an \`Edit\` on) PLUS its caller/callee trail — the same bytes Read gives you, plus who calls it and what your change would break, for fewer tokens. For an OVERLOADED name it returns EVERY matching definition's body in one call, so you never Read a file to find the right overload. Or pass a FILE PATH alone (no symbol) to get that whole file's symbol map + what depends on it — a Read replacement for a source file
+- **Reading a source FILE (any time you'd use the \`Read\` tool)** → \`codegraph_node\` with a \`file\` path and no \`symbol\`. It returns the file's **current source with line numbers — the same \`<n>\\t<line>\` shape \`Read\` gives you, safe to \`Edit\` from** — narrowable with \`offset\`/\`limit\` exactly like \`Read\`, PLUS a one-line note of which files depend on it. Same bytes as \`Read\`, faster (served from the index), with the blast radius attached. Use it **instead of \`Read\`** for indexed source files; fall back to \`Read\` only for what codegraph doesn't index (configs, docs). Pass \`symbolsOnly: true\` for just the file's structure.
+- **About to read or edit a symbol you can name** → \`codegraph_node\` with that \`symbol\` (SECONDARY — the after-explore depth tool): the verbatim source (\`includeCode: true\`) PLUS its caller/callee trail, so before changing it you see what calls it and what your edit would break. For an OVERLOADED name it returns EVERY matching definition's body in one call, so you never Read a file to find the right overload
 - **"What's in directory X?"** → \`codegraph_files\`
 - **"Is the index ready / what's its size?"** → \`codegraph_status\`
 
@@ -65,7 +66,7 @@ typically one to a few calls; a grep/read exploration is dozens.
 - **Don't grep first** when looking up a symbol by name — \`codegraph_search\` is faster and returns kind + location + signature.
 - **Don't chain \`codegraph_search\` + \`codegraph_node\`** to understand an area — ONE \`codegraph_explore\` returns the relevant symbols' source together in a single round-trip.
 - **Don't loop \`codegraph_node\` over many symbols** — one \`codegraph_explore\` call returns them all grouped by file, while each separate call re-reads the whole context and costs far more. Use \`codegraph_node\` for a single symbol.
-- **Don't \`Read\` a file just to see or edit a symbol you can name** — \`codegraph_node\` returns the same current source plus its caller/callee trail in one call, for fewer tokens. Reach for raw \`Read\` only for what codegraph doesn't index (configs, docs) or when the staleness banner flags a file as pending re-index.
+- **Don't reach for the \`Read\` tool on an indexed source file** — \`codegraph_node\` with a \`file\` reads it for you (same \`<n>\\t<line>\` source, \`offset\`/\`limit\` like Read, faster, with its blast radius), and with a \`symbol\` it returns the source plus the caller/callee trail. Reach for raw \`Read\` only for what codegraph doesn't index (configs, docs) or when the staleness banner flags a file as pending re-index.
 - **After editing, check the staleness banner.** When a tool response starts with "⚠️ Some files referenced below were edited since the last index sync…", the listed files are pending re-index — Read those specific files for accurate content. Every file NOT in that banner is fresh, so still trust codegraph. \`codegraph_status\` also lists pending files under "Pending sync".
 
 ## Limitations