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/handler/comment.go

@@ -9,6 +9,7 @@ import (
 	"github.com/go-chi/chi/v5"
 	"github.com/jackc/pgx/v5/pgtype"
 	"github.com/multica-ai/multica/server/internal/logger"
+	"github.com/multica-ai/multica/server/internal/mention"
 	"github.com/multica-ai/multica/server/internal/util"
 	db "github.com/multica-ai/multica/server/pkg/db/generated"
 	"github.com/multica-ai/multica/server/pkg/protocol"
@@ -130,6 +131,9 @@ func (h *Handler) CreateComment(w http.ResponseWriter, r *http.Request) {
 	// Determine author identity: agent (via X-Agent-ID header) or member.
 	authorType, authorID := h.resolveActor(r, userID, uuidToString(issue.WorkspaceID))
 
+	// Expand bare issue identifiers (e.g. MUL-117) into mention links.
+	req.Content = mention.ExpandIssueIdentifiers(r.Context(), h.Queries, issue.WorkspaceID, req.Content)
+
 	comment, err := h.Queries.CreateComment(r.Context(), db.CreateCommentParams{
 		IssueID:     issue.ID,
 		WorkspaceID: issue.WorkspaceID,