fix(github): remove dead MySQL DROP INDEX guidance from plan comments

[aparajon]

Spirit's unsafe classification treats index drops as safe (no data loss), so unsafe-change reasons never contain a "DROP INDEX" fragment — index drops surface through the invisible_index_before_drop linter, whose message carries no such fragment either. That made the plan-comment guidance branch keyed off counting "DROP INDEX" in unsafe reasons reachable only from a hand-written preview fixture, never from a real plan.

• Remove the dead guidance branch and its unsafeDropIndexUsageTargets helper from the plan-comment renderer; writeUnsafeWarning/writeUnsafeDropGuidance drop their now-unused isMySQL parameter.
• Remove the Drop Index Blocked preview scenario and its CLI registrations, and the tests asserting the removed guidance.
• The DROP COLUMN / DROP TABLE application-usage guidance is unchanged and keeps a rendered-output test of its own.
• TEMPLATES.md regenerated; the regen also picks up the cancelled-shard glyph from an earlier change that had not been regenerated into the file.

Drop Column Blocked (guidance that remains, unchanged)

Schema Change Plan — Staging

Database: testapp | Type: MySQL | Schema Name: testapp

Requested by @jackjackbits at 2026-01-01 00:00:00 UTC · planned from abcdef1

ALTER TABLE `customers` DROP COLUMN `nickname`;

📋 Plan: 1 table to alter

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⛔ Apply rejected: 1 unsafe change detected

• customers: Unsafe operation detected: DROP COLUMN nickname

Destructive drop guidance:

Before allowing a destructive drop, first deploy application code that no longer reads from or writes to the dropped column.

🚨 To proceed with these destructive changes, re-run with --allow-unsafe:

schemabot apply -e staging --allow-unsafe

This PR was written by Claude Code (Claude Fable 5).

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Removes unreachable “DROP INDEX” destructive-drop guidance from the GitHub plan/blocked-unsafe comment rendering, along with the associated preview scenario and tests, because real unsafe-change reasons do not include DROP INDEX.

Changes:

• Removed the dead DROP INDEX guidance branch and helper (unsafeDropIndexUsageTargets) from the plan comment renderer; simplified writeUnsafeWarning/writeUnsafeDropGuidance signatures accordingly.
• Removed the “Drop Index Blocked” preview scenario and all CLI preview registrations for it.
• Regenerated TEMPLATES.md to drop the removed scenario and to reflect the updated cancelled-shard glyph.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
TEMPLATES.md	Removes the “Drop Index Blocked” rendered template; updates cancelled glyph rendering in regenerated output.
pkg/webhook/templates/preview.go	Deletes the PreviewCommentDropIndexBlocked fixture.
pkg/webhook/templates/plan.go	Removes dead drop-index guidance logic; simplifies unsafe-warning/guidance helpers and call sites.
pkg/webhook/templates/apply_test.go	Deletes tests for removed drop-index guidance; keeps drop-column guidance test.
pkg/webhook/templates/apply_commands.go	Updates call to writeUnsafeDropGuidance after signature change.
pkg/cmd/internal/templates/preview.go	Removes the comment_drop_index_blocked preview type constant.
pkg/cmd/internal/templates/preview_dispatch.go	Removes CLI dispatch case for the deleted drop-index preview.
pkg/cmd/internal/templates/preview_comment.go	Removes the drop-index preview from “preview all comment templates” output lists.
pkg/cmd/commands/preview.go	Removes the drop-index preview from the supported preview set and help text.

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