feat: one cli-core without an override, Telegram's deletion rule and skill on the Messenger, the contract cases

[leemour]

Task

Plan items 3 and 10 of cli-messaging's "port for N messengers" plan, and one copy of cli-core without the pnpm override. #211 merged the cli-messaging 0.98.0 bump while this PR was being built, so this PR is rebased onto #211 and no longer bumps anything.

Why

• cli-messaging 0.98.0 takes @leemour/cli-core as a peer dependency (>=0.13.0 <0.16.0). With that, the override that forced one copy is no longer needed.
• The store applies Telegram's rule for a deletion that names no chat by checking the provider's name. The plan moves that rule into tg's Messenger, so the breaking release can remove the Telegram branch from the store.
• The MCP server serves a CLI's SKILL.md as <cli>://skill only when the Messenger names it.
• cli-messaging ships contract cases for an adapter; tg had never run them.

What

• The overrides entry for cli-core is removed. tg's 0.15.0 pin (from fix(docs): commands that change only this computer get their own label #211) is inside the peer range, and pnpm why @leemour/cli-core finds one version. tg commands --json is byte-for-byte the same as main's, including all 56 mutates: true marks and the 12 local marks.
• TELEGRAM.deletedWithoutChat: the predicate is the same rule the store applies today. The store's two tests for that rule are ported to src/commands/context.test.ts and run with the predicate. A table covers channel, gigagroup, monoforum, a chat stored by its id alone, and Saved Messages.
• TELEGRAM.skill = new URL("../../skills/tg-cli/SKILL.md", import.meta.url), exported as SKILL; tg skill uses the same constant. The file resolves from dist/commands/context.js (checked after pnpm build).
• src/telegram/adapter.contract.test.ts runs contractCases over TelegramAdapter. A seeded stand-in for mtcute's TelegramClient holds the seed with Telegram's id shapes: a one-to-one chat's id is the person's, and a supergroup's starts with -100. It keeps what is sent and drops a repeated random_id. It pages with mtcute's own getHistory logic over Telegram's messages.getHistory offsets. 22 pass, 1 skipped: feed, because tg has no PushedHistory.
• Fix found by the suite: contact left out the one-to-one chat, because Telegram's common chats are groups only. That chat is now in the list, newest first with the rest. The changelog has it under Fixed.

Testing

• pnpm lint && pnpm typecheck && pnpm test: 439 passed, 1 skipped.
• pnpm test:matrix, pnpm docs:check, pnpm parity:check, pnpm smoke:bun, pnpm version:check and pnpm install --frozen-lockfile all pass. docs/commands.md regenerates unchanged.
• A mutation check: breaking the + 1 in around's offset makes the around case fail.
• Nothing was run against live Telegram.

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