fix(errors): improve config error messages with section paths and remediation hints

Improve vague error messages in channel initialization and tool setup
to include specific config key paths and remediation steps, matching
the quality standard set by proxy validation errors.

Changes:
- telegram.rs: Include [channels.telegram] section path and required
  fields (bot_token, allowed_users) in missing-config error; add
  onboard hint; specify channels.telegram.allowed_users in pairing
  message; improve parse error context
- whatsapp.rs: Specify channels.whatsapp.allowed_numbers key path
  in unauthorized-number warning
- linq.rs: Specify channels.linq.allowed_senders key path in
  unauthorized-sender warning; add onboard hint
- web_search_tool.rs: Include tools.web_search.provider config path
  and valid values in unknown-provider error

Addresses API surface audit §8.2 (config context in error messages).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/tools/web_search_tool.rs

@@ -219,7 +219,10 @@ impl Tool for WebSearchTool {
         let result = match self.provider.as_str() {
             "duckduckgo" | "ddg" => self.search_duckduckgo(query).await?,
             "brave" => self.search_brave(query).await?,
-            _ => anyhow::bail!("Unknown search provider: {}", self.provider),
+            _ => anyhow::bail!(
+                "Unknown search provider: '{}'. Set tools.web_search.provider to 'duckduckgo' or 'brave' in config.toml",
+                self.provider
+            ),
         };
 
         Ok(ToolResult {