feat: make --ext override PAPI builtin extensions

[peetzweg]

Closes #171.

Makes --ext a generic override that applies to every signed extension a chain declares, including polkadot-api builtins.

Why: --asset worked on the ChargeAssetTxPayment builtin, but naming any other builtin (e.g. ChargeTransactionPayment) in --ext was silently ignored — the two flags were inconsistent and --ext (the more generic one) couldn't reach builtins at all.

How: in buildCustomSignedExtensions an explicit --ext entry now takes priority over the builtin skip, so a named builtin is passed to polkadot-api's customSignedExtensions; un-named builtins are still auto-filled. --asset is now just sugar over an --ext override of ChargeAssetTxPayment (the bespoke skip-set juggling is gone). One judgment call: overriding builtins is a power-user escape hatch — the ergonomic flags (--nonce/--tip/--mortality/--asset) remain the normal path.

Example (real, executable — the detail view now advertises the override for builtins):

$ dot polkadot.extensions.CheckMortality

CheckMortality (Transaction Extension)

  Value type:       enum(256 variants)
  AdditionalSigned: [u8; 32]
  Handled by:       polkadot-api (builtin) — filled in automatically, override with --ext

Usage:
  dot polkadot.tx.<Pallet>.<Call> --from <acc> --ext '{"CheckMortality":{"value":<v>}}'
  Builtin: polkadot-api sets a value by default; pass --ext only to override it.

Force an immortal era on any call by overriding the builtin:

dot polkadot.tx.System.remark 0xdeadbeef --from alice \
  --ext '{"CheckMortality":{"value":{"type":"Immortal","value":null}}}'

Tests + changeset added; docs, skill and README updated. Full suite (1742 tests), lint, typecheck and build all green.