chore(mcp): migrate to st (Phase 4 of coord→smalltalk/st rename) (#46)

Switches pty's MCP server registration from `coord` to `st`, the
canonical short name from brief-005. Binary path moves from `bin/coord`
(back-compat shim) to `bin/st` (canonical entry).

Both forms continue to work post-Phase-0 — the coord repo's PR #14
shipped a 5-alias backward-compat layer. This commit just opts pty into
the canonical names now that they exist. Takes effect on next
pty-claude restart.

Local-only configs (pty.toml `ST_IDENTITY`, hook paths) are gitignored
and updated out-of-band on each machine.