feat: add dimmer_set subcommand (API 2.0)

[chicks-net]

What exists already

• locatorCommands (main.go:88) maps subcommand names to wire bytes, but has no
  dimmer entry
• sendCommand sends fixed 3-byte mode commands; sendSetCommand sends the
  6-byte SetTimer struct — neither fits a 1-byte-payload command

What's missing

Per TimeMachinesCorp Locator Protocol API v2.0
section 1.4.4, Dimmer Set is:

Byte	Description
0	Command byte: 0xB5
1	Digit Brightness: 0-100

There is no CLI subcommand to send it.

Proposed subcommand

• ctm dimmer_set <level> <clock_ip> → sends \xb5 + one brightness byte

Example:

ctm dimmer_set 50 192.168.42.204   # sends \xb5\x32, expects ACK

Notes / constraints

• API v2.0-only feature; the clock must return a 40-byte status response to be
  v2.0, though the command itself just needs an ACK.
• Volatile: the dimmer level is not saved to non-volatile memory — it is
  lost on reboot or until changed. Worth noting in --help text.
• Needs a small send helper for a command byte + 1 payload byte (or a
  generalized payload-send function that sendSetCommand can share).
• Brightness must be validated to 0-100.

Acceptance Criteria

• locatorCommands["dimmer_set"] = "\xb5" entry added and pinned in main_test.go
• ctm dimmer_set 50 <ip> sends \xb5\x32 and receives ACK
• levels outside 0-100 are rejected with a clear error
• dispatch test via newCommandTree with fakeConn verifying wire bytes
• README.md and CLAUDE.md subcommand lists updated (remove from "coming soon")