fix: pre-release agent and config fixes

[mikolajbadyl]

Description

• Fixes UI status bar to read the Active LLM Config directly, honoring environment variables (like OCTRAFIC_MODEL) instead of strictly reading the user's config file.
• Removes legacy fallback logic from openai/claude/gemini clients to ensure they strictly respect the settings passed from the main config package.
• Adds missing 'ExportTests' tool to the agent's system prompt documentation.

[kilo-code-bot]

Code Review Summary

Status: 4 Issues Found | Recommendation: Address before merge

Overview

Severity	Count
CRITICAL	0
WARNING	4
SUGGESTION	0

Issue Details (click to expand)

WARNING

File	Line	Issue
internal/config/config.go	141	Breaking change: Removed fallback to provider-specific env vars (ANTHROPIC_API_KEY, OPENAI_API_KEY). Users must now use OCTRAFIC_API_KEY.
internal/llm/claude/client.go	64	Client no longer falls back to ANTHROPIC_API_KEY. Must use OCTRAFIC_API_KEY.
internal/llm/gemini/client.go	77	Client no longer falls back to GOOGLE_API_KEY. Must use OCTRAFIC_API_KEY.
internal/llm/openai/client.go	196	Client no longer falls back to OPENAI_API_KEY. Must use OCTRAFIC_API_KEY.

Other Observations (not in diff)

File	Line	Issue
N/A	-	No issues found outside the diff

Files Reviewed (7 files)

• internal/agents/chat.go - Added ExportTests documentation (no issues)
• internal/cli/tui.go - Changed to use GetActiveLLMConfig() (behavior change, appears intentional)
• internal/config/config.go - 1 issue
• internal/llm/claude/client.go - 1 issue
• internal/llm/claude/provider.go - Updated to pass baseURL (no issues)
• internal/llm/gemini/client.go - 1 issue
• internal/llm/openai/client.go - 1 issue

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Summary: This PR makes a breaking change by removing support for provider-specific environment variables (ANTHROPIC_API_KEY, OPENAI_API_KEY, GOOGLE_API_KEY) in favor of a unified OCTRAFIC_* prefix. Users will need to update their environment configuration. Consider adding migration documentation or a compatibility layer.