feat(s071): the Telegram gateway, and two fixes to the gateway core

[HackTuah]

Slice 071, the Telegram gateway, plus two security fixes to slice 070's gateway core. The slice stays in_progress until the owner's live run with a real bot token (steps in the slice's PROOF).

Read first: two fixes to 070's approved core

• F1, the pairing code leaked to strangers. Router.prompt_for/1 sent an unknown sender the code they were supposed to read on /gateways, so anyone who found the bot could pair by echoing it back. 070's own test asserted the leak. The prompt now says where the code is, never what it is. This restores docs/07-security-model.md's rule, "shown on /gateways and nowhere else". Nothing was exposed before this PR, because no gateway was ever started.
• F2, ambiguous short approval ids. UUIDv7 ids share their 8-character prefix for about 65 seconds, so /approve <short> could decide the wrong request. Ambiguous prefixes are now refused, and buttons carry the full id.

The gateway

• Bot API over Req, which is already in the lock. telegex has only an old release candidate, and ex_gram's in-memory offset fails AC5 by construction.
• Token handling: the token is read on every call and never stored. Errors never carry the URL, which contains the token, and Req's retries are off. The log filter and the secret scanner learn the token's shape.
• MarkdownV2: conversion that only emits closed entities, chunked after conversion. Two properties check it against an independent parse checker.
• Delivery:
  • edits throttled to at least 1 s, keeping the final text;
  • long polling with the offset persisted before routing (at most once);
  • photos fetched only after admission, size-capped and magic-checked, stored by digest;
  • approval buttons only under the channel cap.
• Trinity.Gateways.Supervisor with TRINITY_GATEWAYS. Nothing in the app started a gateway before this slice.
• Images reach the session row and a :vision model, or are replaced by a note for any other model.

Criteria

AC	State
AC4 formatting, escaping, chunking	proven, including properties
AC5 crash and restart, no duplicate	proven
AC1, AC2, AC3, AC6	automatic halves proven against the fake; live runs queued for the owner

Evidence

• Author's gate: exit 0, 1015 passed (22 properties, 993 tests), 18 excluded, prod_check: PASS.
• Reviewer re-run before commit: the 10 changed and new test files, 96 passed (2 properties, 94 tests). mix format --check-formatted OK; mix credo --strict --all found no issues.

Merge note

Slice 072 (Mattermost) builds on the same core and is aligning with this PR's supervisor and adapter shapes. Merge this one first.