docs: complete API reference and CONTRIBUTING guide (Phase 2)

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Phase 2 — API Reference & CONTRIBUTING Guide

This PR completes the final two Phase 2 documentation tasks.

docs/api.md — Full API Reference (new file)

Covers every public type and function in both the top-level olapsql package and the api/types sub-package:

• Configuration — fields, both usage modes (with/without live DB)
• Manager — NewManager, RunSync, RunChan, BuildSQL, BuildTransaction, SetLogger, GetClients, GetDictionary; each with parameters, return values, and a code example
• Clients / ClientsOption — all methods: RegisterByKV, RegisterByOption, SetLogger, Get, BuildDB, BuildSQL, NewClients
• DBOption — DSN and Type fields
• Dictionary / Option / AdapterOption — schema loading
• Package-level helpers — RunSync, RunChan, BuildResultSync, BuildResultChan
• Query — all fields with descriptions; Go struct and JSON construction examples
• TimeInterval — semantics and generated SQL
• Filter — all FilterOperatorType constants with SQL equivalents, leaf vs. tree usage, examples
• Metric — all MetricType constants with SQL patterns; TOML composition example
• Dimension — all DimensionType constants
• OrderBy / Limit / Result — fields and iteration example
• DBType — all supported backends

CONTRIBUTING.md — Complete Contributor Guide (replaces stub)

• Prerequisites (Go 1.22, SQLite; ClickHouse not required for tests)
• Fork + clone + sync workflow
• Full project layout table with one-line descriptions
• Running tests (all, single package, pattern, coverage)
• Writing tests guide
• Code style rules (gofmt, godoc, any over interface{}, no panic in lib code)
• Commit message convention (Conventional Commits)
• PR checklist
• Step-by-step instructions for adding: new DB backend, new metric type, new filter operator
• Releases and versioning policy (SemVer, breaking-change PR requirements)

Part of

Phase 2: Documentation — spec

With this PR, all Phase 2 tasks are complete.