fix(installer): tell Kiro IDE users to enable MCP in Settings (#475)

PR #473 emitted only "Restart Kiro for MCP changes to take effect."
That note is incomplete for Kiro IDE users: the IDE ships with MCP
support disabled by default, so a freshly-written
~/.kiro/settings/mcp.json is ignored until the user opens Settings,
searches "MCP", and flips the "Kiro Agent: Configure MCP" dropdown to
"Enabled". The agent then reports "No MCP powers installed" and falls
back to grep/Read — which looks like an installer wiring bug but isn't.

Kiro CLI doesn't gate on this flag — it reads the same file without
any toggle — so the second note calls out which audience needs the
extra step.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>