Remove deprecated `dot parachain` command

Background

The dot parachain <paraId> command was deprecated in the PR that introduced sovereign-account derivation flags on dot account (feature-pallet-accounts branch). Equivalent functionality lives at:

• Stateless lookup (script-friendly): dot account inspect --parachain <id> --parachain-type <child|sibling> (add --prefix <N> and/or --json as before)
• Stored (named, reusable as --from, in tx args, in dot account list): dot account add <name> --parachain <id> --parachain-type <child|sibling>

dot parachain is currently kept verbatim for backward compatibility with production scripts that pin the older CLI surface. It now prints a one-line deprecation warning to stderr only on each invocation; stdout is byte-identical to the pre-deprecation behavior so existing pipes / jq / parsers keep working.

Scope of removal

When this issue is worked, delete:

• src/commands/parachain.ts — the command itself
• src/commands/parachain.test.ts — the contract tests for the legacy interface
• The import { registerParachainCommand } from \"./commands/parachain.ts\" line in src/cli.ts
• The registerParachainCommand(cli); call in src/cli.ts
• The parachain line in printHelp() in src/cli.ts

The shared core helpers in src/core/parachain.ts (deriveSovereignAccount, isValidParaId, SOVEREIGN_ACCOUNT_TYPES, SovereignAccountType) must remain — they are reused by dot account add / dot account inspect for the supported derivation flags.

A changeset entry should accompany the removal (major if we treat removing a deprecated CLI command as breaking, otherwise minor per repo convention).

Pre-removal checklist

• Confirm no internal scripts / CI / docs still call dot parachain (grep the org).
• Confirm a release containing the deprecation warning has been out long enough that consumers have had reasonable notice.
• Update README / docs / SKILL.md to drop any remaining references to dot parachain (the deprecation PR may have already removed most).

Migration cheat sheet for downstream

Note: the new path requires --parachain-type explicitly (no implicit child+sibling pair). Scripts that consumed both halves of the old default output should issue two inspect calls.

Start with src/commands/parachain.ts and src/commands/parachain.test.ts, then inspect the registration and help entries in src/cli.ts. Grep the repository for dot parachain references and review README, docs, and SKILL.md. Done means the legacy command and its registrations are removed, shared helpers remain, references are updated, and a changeset accompanies the removal.