fix: include docs/ in wheel and look up bundled copy at runtime

[mvanhorn]

Summary

get_docs_section (used by docs and suggested_questions MCP tools) only searched filesystem-relative paths. After pipx install code-review-graph==2.3.2, the wheel didn't include docs/LLM-OPTIMIZED-REFERENCE.md, so both tools returned status: error, reason: docs_not_found outside a checked-out repo.

Two changes:

1. pyproject.toml: tool.hatch.build.targets.wheel.force-include ships docs/LLM-OPTIMIZED-REFERENCE.md to code_review_graph/docs/LLM-OPTIMIZED-REFERENCE.md inside the wheel.
2. tools/docs.py: when no repo_root is passed, check the in-package code_review_graph/docs/ first; fall back to source-tree docs/ next to the package. Both paths now resolve.

This PR is scoped to bug 1 of #480; bug 2 (suggested_questions returning role-user dicts) is already addressed in 4b6293f.

Why this matters

#480 surfaces a hard failure for pipx/uvx users on 2.3.2. The wheel layout is the root cause — tool.hatch.build.targets.wheel.packages = ["code_review_graph"] doesn't pick up top-level docs/. Force-include in pyproject.toml ships the file in the wheel at a path the runtime can find. 2 new tests assert both the source-tree path and the bundled-wheel path resolve from outside any repo.

closes #480

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