Remove obsolete sequential ID and collision resolution docs

- Remove docs/collision-resolution-failure-analysis.md (sequential ID collision issues no longer exist)
- Remove commands/renumber.md (renumber command removed, hash IDs don't need renumbering)

These docs are obsolete since v0.20.1 uses hash IDs by default.

Amp-Thread-ID: https://ampcode.com/threads/T-0b000145-350a-4dfe-a3f1-67d4d52a6717
Co-authored-by: Amp <amp@ampcode.com>

commands/renumber.md

@@ -1,29 +0,0 @@
----
-description: Renumber all issues sequentially
-argument-hint: [--dry-run] [--force]
----
-
-Renumber all issues sequentially to eliminate gaps in the ID space.
-
-## What It Does
-
-- Renumber all issues starting from 1 (keeps chronological order)
-- Update all dependency links (all types)
-- Update all text references in descriptions, notes, acceptance criteria
-- Show mapping report: old ID -> new ID
-- Export updated database to JSONL
-
-## Usage
-
-- **Preview**: `bd renumber --dry-run`
-- **Apply**: `bd renumber --force`
-
-## Risks
-
-⚠️ **Warning**: This operation cannot be undone!
-
-- May break external references in GitHub issues, docs, commits
-- Git history may become confusing
-- Backup recommended before running
-
-Use sparingly - only when ID gaps are problematic.