feat(mentions): support @mentioning issues + server-side auto-expansion (#242)

* feat(mentions): support @mentioning issues in comments

- Extend MentionItem type to include "issue" alongside "member"/"agent"
- Add issue search (by identifier and title) to mention suggestion dropdown
- Render issue mentions with CircleDot icon in autocomplete popup
- Issue mentions serialize as [MUL-117 Title](mention://issue/id) (no @ prefix)
- Markdown renderer shows issue mentions as clickable links to /issues/:id
- Backend mentionRe regex updated to match issue mention type

* feat(mentions): auto-expand issue identifiers and add mention format to agent instructions

1. Path A — CLAUDE.md template (runtime_config.go):
   Add a "## Mentions" section teaching agents the mention serialization
   format for issues, members, and agents. All agents automatically
   receive this via the auto-generated CLAUDE.md.

2. Approach 2 — Server-side auto-conversion (internal/mention/):
   New ExpandIssueIdentifiers() utility that scans comment content for
   bare issue identifiers (e.g. MUL-117) and replaces them with
   [MUL-117](mention://issue/<uuid>) mention links. Skips code blocks,
   inline code, and existing markdown links. Integrated into both:
   - handler.CreateComment (HTTP API path)
   - service.createAgentComment (agent task output path)

server/internal/daemon/execenv/runtime_config.go

@@ -126,6 +126,13 @@ func buildMetaSkillContent(provider string, ctx TaskContextForEnv) string {
 		b.WriteString("\n")
 	}
 
+	b.WriteString("## Mentions\n\n")
+	b.WriteString("When referencing issues or people in comments, use the mention format so they render as interactive links:\n\n")
+	b.WriteString("- **Issue**: `[MUL-123](mention://issue/<issue-id>)` — renders as a clickable link to the issue\n")
+	b.WriteString("- **Member**: `[@Name](mention://member/<user-id>)` — renders as a styled mention and sends a notification\n")
+	b.WriteString("- **Agent**: `[@Name](mention://agent/<agent-id>)` — renders as a styled mention\n\n")
+	b.WriteString("Use `multica issue list --output json` to look up issue IDs, and `multica workspace members --output json` for member IDs.\n\n")
+
 	b.WriteString("## Output\n\n")
 	b.WriteString("Keep comments concise and natural — state the outcome, not the process.\n")
 	b.WriteString("Good: \"Fixed the login redirect. PR: https://...\"\n")