docs: comprehensive Go SDK Guide — complete API.md rewrite

Full rewrite of docs/API.md from a brief reference into a thorough
SDK guide for engineers building on odek as a Go library:

- Quickstart: import, configure, run in 10 lines
- Architecture overview: ReAct loop flow diagram
- Core types deep-dive: Config (all fields), Tool (all 4 methods),
  Agent (all 6 methods with return types), ModelProfile, KnownProfiles
- Single-shot tasks: basic usage + custom tools + system prompts
- Multi-turn sessions: RunWithMessages, session persistence patterns
- Custom tools walkthrough: 5-step guide with real gitLog example
- Tool patterns table: 5 production patterns (zero-alloc, graceful errors, etc.)
- Memory system: AddFact, ReplaceFact, RemoveFact, BufferLines, episodes
- Model profiles: built-in table, custom profile registration, lookup
- Token tracking: per-call and session-level patterns
- Error handling: creation failures, run errors, tool error strategy
- Complete example: 100-line agent with line_count + slack_notify tools
- Package reference: all 4 exported functions, 4 types, 1 const, 1 var
- Import path + migration guide from v0.14.x

Updated README.md and landing page doc links to reference 'Go SDK Guide'

Author: jkyberneees

README.md

@@ -147,7 +147,7 @@ odek run "@README.md what does this project do?"
 |-----|--------|
 | [CLI Reference](docs/CLI.md) | All commands, subcommands, flags, error codes |
 | [Configuration](docs/CONFIG.md) | Config files, env vars, priority chain, all sections |
-| [Programmatic API](docs/API.md) | Go library: Tool interface, Config, Agent, Run, model profiles |
+| [Programmatic API](docs/API.md) | **SDK Guide**: import, Agent lifecycle, Tool interface, multi-turn sessions, memory system, model profiles, complete examples |
 | [Providers & Models](docs/PROVIDERS.md) | Supported providers, thinking config, context windows |
 | [Memory](docs/MEMORY.md) | Three-tier design, go-vector merge-on-write, `memory` tool |
 | [Sessions](docs/SESSIONS.md) | Multi-turn conversations, save/resume/trim/cleanup |