When an admin lands a vision pull, only the fenced vision block of the direction is rewritten, and the owner's own words are kept word for word #5834

The second half of #655, with every finding from sentinel's three reviews of #5761 built in.

Done when:

• [ ] The merge follow-up acts only on a pull its placement marks as a vision pull, never on a branch name.
• [ ] It writes only when the merger canAdmin the repository. It never acts as autonomyBy or any other account.
• [ ] The direction gains a block fenced by start and end markers. It is regenerated from VISION.md's purpose, order of work and rules, and the whole direction stays under 2,000 characters. Everything outside the block is kept byte for byte.
• [ ] A start marker with no end marker leaves the direction unchanged and logs why. Nothing the owner wrote is dropped.
• [ ] A non-admin merge leaves the direction unchanged. It posts no comment under anyone's name; it writes a repo event that the pull page renders as the forge's line.
• [ ] A repo.settings_updated event lists autonomyDirection when the block changed.
• [ ] Tests cover an admin merge, a writer merge, an orphan start marker, an owner's text before and after the block, and the 2,000-character bound.