docs(skills): esp-target-debug — Espressif built-in USB-JTAG backend, rig-verified

P4 (hand-wired USB-SJ breakout, GPIO24/25 from header J1):
- COEXISTENCE verified: 303a:1001 + cafe:4008 enumerated simultaneously;
  gdb attach/halt/bt during live CDC traffic; symbolized app backtrace
  (tud_task_ext <- usb_device_task <- vPortTaskWrapper)
- set ESP_RTOS FreeRTOS before board cfg -> full dual-core task list;
  without it, bare 'Remote target'
- attach-may-reset nuance flagged (post-mortem autopsy caution)
- console = UART0 (CP2102 flasher tty) on stock builds; D+/D- swap symptom
  documented (low-speed + error -71 vs full-speed)

S3 (same-port PHY swap):
- boundary captured live in dmesg: same hub port flips 303a:1001 ->
  cafe:4008 as the app boots; openocd 'esp_usb_jtag: could not find or open
  device!' verbatim
- attach/halt/symbol resolution verified via board_test (usb_new_phy absent
  from ELF when CFG_TUD/TUH=0); app-context keep-alive quirk (~4 s
  unattended drop, -71 half-dead, UART esptool reset recovers); cpu1
  OCD_ID=0 -> ESP_ONLYCPU=1; telnet-halt + gdb-read scripted pattern;
  RTC_CNTL PHY-mux reference (0x60008120) + esptool read_mem/write_mem
- target-debug pointer + target-debugger agent table row

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