Vendor the server code into the client plugin folder

[HarperZ9]

Changes

• client-plugin/server/src/index_graph/: 120 vendored files, the same selection and bytes the source ZIP places under server/src/.
• scripts/build_client_package.py: vendored_payload() and plugin_entries() reuse the builder's own entries() selection; --sync-vendored rewrites the folder from src/ and deletes stale files. Build inputs exclude the vendored copy, so the ZIP takes server/src from src/ once.
• client-plugin/server/serve.py: no fallback to the repository's src. A missing copy prints index: the server code is missing from the plugin folder. Reinstall the plugin. and exits 1. -I -S -B unchanged.
• client-plugin/.claude-plugin/icon.png: now the 512 px canon PNG (99 KiB). The 1024 px file is 273 KiB, over the 256 KiB per-file limit, and lossless recompression only reached 268 KiB.
• README install text says the folder carries its own server code. The "What this plugin runs and handles" facts are unchanged. The Git environment-copy code is unchanged.
• CHANGELOG Unreleased entry. No version bump.

Tests

• Drift test against the builder (missing, extra or changed file, CRLF normalized), naming the sync command.
• Isolated launch: copies only client-plugin/, runs the exact .mcp.json command with ${CLAUDE_PLUGIN_ROOT} and ${user_config.*} substituted, checks initialize and tools/list, then removes server/src and checks the one-line message and exit 1.
• Limits: 131 files (limit 512), no file at or above 256 KiB (largest 99 KiB icon, largest code file router_jobs.py 41 KiB), no .gitattributes in the folder.
• python -m pytest: 937 passed. Manual check from a fresh clone of this branch with only the plugin folder returned 6 tools. claude plugin validate passes with the five directory-field warnings.

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