Step 3 gaps: file_edit tool, web_search tool, output scrubbing

## Context

Step 3 (Tool System) shipped with 5 built-in tools but the MVP spec calls for 8. Three items are missing.

## Missing tools

### file_edit (SideEffecting)
Find-and-replace edit in a file. The LLM specifies old text and new text, and the tool replaces it. More surgical than write_file for small changes.

Parameters:
- `path` (string, required) — file path
- `old_text` (string, required) — text to find
- `new_text` (string, required) — replacement text

### web_search (SideEffecting)
Search the web. MVP supports DuckDuckGo (no API key needed), with Google and SearxNG as optional backends.

Parameters:
- `query` (string, required) — search query
- `num_results` (integer, optional, default 5) — number of results

Config already exists: `tools.web_search.provider`, `tools.web_search.base_url`, `tools.web_search.api_key_env`

## Missing feature

### Output scrubbing
Applied to all tool outputs before returning to LLM:
1. **Path scrubbing** — replace host absolute paths with workspace-relative paths
2. **Keyword scrubbing** — redact known secret patterns (API keys, tokens)
3. **Entropy-based detection** — flag high-entropy strings (Shannon >= 3.8)

## Also noted in MVP but deferred
- WASM sandbox (wazero) — the MVP spec mentions it but process-mode tools work fine for launch. Can be added later for defense-in-depth.