[DISCORDSH] Implement Notice Board embed generator

[h0lybyte]

Parent

Part of #7849

Description

Implement the Notice Board embed generator that surfaces blockers, stagnation alerts, and cross-department coordination issues as rich Discord embeds.

Example Output

notice_embed = discord.Embed(
    title="[Blocker] AI Pathfinding Mismatch",
    description="Doorway dimensions and corridor widths are preventing coherent navmesh generation.",
    color=0xE67E22,
    timestamp=datetime.now(timezone.utc)
)
notice_embed.add_field(name="Ticket ID", value="`NB-0007`", inline=True)
notice_embed.add_field(name="Priority", value="`HIGH`", inline=True)
notice_embed.add_field(name="Stagnation", value="`3 days`", inline=False)

Acceptance Criteria

• Generates embed from GitHub issue/PR data
• Includes ticket ID, department, priority, reporter, stagnation info
• Color-coded by priority (HIGH=orange, CRITICAL=red, etc.)