fix(uninstall): remove an instruction block written by any past release

_remove_instruction matched only the section text the running version
generates. Install can now upgrade a block from any release, so leaving
removal on a single exact text meant a user who asked for the instructions to
go kept whichever wording their original install wrote, reported as "differs
from a known installed section" and silently left in place.

Removal now walks skills._known_instruction_sections(), the same recorded
texts install matches, longest first so a short variant cannot strand the tail
of a longer one that contains it. That covers every past release including the
variant a markdown formatter rewrote in this repo's own files, and clears
duplicate blocks an older release stacked up.

The safety rules are unchanged: exact match only, no end boundary is ever
guessed for a block that has no closing marker, and a section that matches
nothing known is left alone and reported rather than deleted. A file holding
both a generated and a hand-edited block loses only the generated one and is
named in the report.

_join_without_instruction closes the gap. Text on both sides is kept and only
the whitespace at the seam is normalised, so removing a block from the middle
of a file leaves one blank line rather than a pile of them, and removing one
appended at the end restores the file byte for byte.

Refs #314, #558

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012fHfGDiZedoxjpKzanHri3