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/channels/linq.rs

@@ -125,7 +125,8 @@ impl LinqChannel {
         if !self.is_sender_allowed(&normalized_from) {
             tracing::warn!(
                 "Linq: ignoring message from unauthorized sender: {normalized_from}. \
-                Add to allowed_senders in config.toml."
+                Add to channels.linq.allowed_senders in config.toml, \
+                or run `zeroclaw onboard --channels-only` to configure interactively."
             );
             return messages;
         }

src/channels/telegram.rs

@@ -387,7 +387,7 @@ impl TelegramChannel {
             .await
             .with_context(|| format!("Failed to read config file: {}", config_path.display()))?;
         let mut config: Config = toml::from_str(&contents)
-            .context("Failed to parse config file for Telegram binding")?;
+            .context("Failed to parse config.toml — check [channels.telegram] section for syntax errors")?;
         config.config_path = config_path;
         config.workspace_dir = zeroclaw_dir.join("workspace");
         Ok(config)
@@ -396,7 +396,11 @@ impl TelegramChannel {
     async fn persist_allowed_identity(&self, identity: &str) -> anyhow::Result<()> {
         let mut config = Self::load_config_without_env().await?;
         let Some(telegram) = config.channels_config.telegram.as_mut() else {
-            anyhow::bail!("Telegram channel config is missing in config.toml");
+            anyhow::bail!(
+                "Missing [channels.telegram] section in config.toml. \
+                Add bot_token and allowed_users under [channels.telegram], \
+                or run `zeroclaw onboard --channels-only` to configure interactively"
+            );
         };
 
         let normalized = Self::normalize_identity(identity);
@@ -691,7 +695,7 @@ impl TelegramChannel {
             } else {
                 let _ = self
                     .send(&SendMessage::new(
-                        "ℹ️ Telegram pairing is not active. Ask operator to update allowlist in config.toml.",
+                        "ℹ️ Telegram pairing is not active. Ask operator to add your user ID to channels.telegram.allowed_users in config.toml.",
                         &chat_id,
                     ))
                     .await;

src/channels/whatsapp.rs

@@ -99,7 +99,8 @@ impl WhatsAppChannel {
                     if !self.is_number_allowed(&normalized_from) {
                         tracing::warn!(
                             "WhatsApp: ignoring message from unauthorized number: {normalized_from}. \
-                            Add to allowed_numbers in config.toml, then run `zeroclaw onboard --channels-only`."
+                            Add to channels.whatsapp.allowed_numbers in config.toml, \
+                            or run `zeroclaw onboard --channels-only` to configure interactively."
                         );
                         continue;
                     }

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 {