docs: document config.yaml

[readwrightexecute]

Summary

• documents ~/.config/lumen/config.yaml / $XDG_CONFIG_HOME/lumen/config.yaml
• explains config precedence: defaults → YAML → env vars → CLI/programmatic overrides → server selection
• documents top-level fields, server fields, env var overrides, known models, aliases, validation rules, and examples
• updates README to link to the canonical configuration doc
• adds config docs tests to verify documented YAML examples parse through the real config loader

Fixes #167.

Tests

• go test ./internal/config -run 'TestDocumentation|TestConfigurationDocumentationIsLinkedFromREADME' -count=1
• go test ./internal/config -count=1
• go test ./cmd -run 'Config|Model|Backend|Index|Search|Stdio' -count=1
• go test ./... -count=1
• git diff --check

Summary by CodeRabbit

• Documentation

  • New CONFIGURATION.md guide documenting YAML config files, environment variable overrides, CLI server selection, and multi-server setups
  • Updated README with consolidated configuration guidance and expanded environment variable reference

• New Features

  • Added support for manutic/nomic-embed-code:7b embedding model

[CLAassistant]

All committers have signed the CLA.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: bbf4de40-c85d-49e3-bba6-3f8f259ec6de

📥 Commits

Reviewing files that changed from the base of the PR and between d0dee0e and da6ef50.

📒 Files selected for processing (3)

• README.md
• docs/CONFIGURATION.md
• internal/config/config_docs_test.go

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📝 Walkthrough

Walkthrough

This PR documents Lumen's YAML configuration system by adding a comprehensive guide (docs/CONFIGURATION.md) covering file locations, precedence rules, schema, environment-variable overrides, CLI filtering, and multi-server failover behavior; updating README.md to cross-reference the new documentation and adding a new supported embedding model; and introducing a test suite that validates all documented configuration examples and parsing behavior.

Changes

Configuration Documentation and Tests

Layer / File(s)	Summary
Configuration overview and schema definition docs/CONFIGURATION.md (lines 1–29, 66–88)	Configuration file locations (e.g., ~/.config/lumen/config.yaml), precedence order across built-in defaults, YAML, environment, and CLI flags, and schema for top-level fields (log_level, max_chunk_tokens, freshness_ttl, reindex_timeout) and per-server fields (backend, host, model, dims, ctx_length, min_score).
Configuration examples and behavior documentation docs/CONFIGURATION.md (lines 31–65, 89–212)	Minimal YAML examples for Ollama and LM Studio, full config with logging/timeout settings, known embedding model metadata table with dims/context/min-score and automatic inference, environment-variable field mappings (with host overrides by backend type), CLI flag filtering (--model/-m, --backend/-b) and failover semantics, multi-server matching example, custom/unknown model handling requiring explicit dims, common validation errors, and MCP server behavior with config reloading.
README configuration section and model registry update README.md (lines 278–309, 327, 337–339)	Rewrote Configuration section to describe persistent YAML with environment-variable overrides limited to the first server and expanded the environment variable reference table; added manutic/nomic-embed-code:7b to supported embedding models; and replaced detailed CLI instructions with a pointer to docs/CONFIGURATION.md for multi-server setups, custom models, and validation rules.
Configuration validation test suite internal/config/config_docs_test.go (lines 1–253)	Added seven test functions covering minimal Ollama/LM Studio configs, full config with log levels and duration parsing, multi-server failover with WithServerSelection, custom model dims requirement, environment-variable override precedence, and README linkage validation; includes utilities for temporary YAML fixtures, environment variable isolation, and repository root detection.

🎯 2 (Simple) | ⏱️ ~10 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'docs: document config.yaml' is specific and directly describes the main change—adding documentation for config.yaml configuration file.
Linked Issues check	✅ Passed	The PR fully addresses issue #167 requirements: documents config.yaml location, structure, precedence, top-level fields (log_level, max_chunk_tokens, freshness_ttl, reindex_timeout), servers with required/optional fields, and provides working examples validated by tests.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to documentation: README.md updates point to new YAML docs, docs/CONFIGURATION.md provides comprehensive configuration guidance, and tests validate documented examples parse correctly.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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