feat: add session-manager plugin for listing, deleting, and cleaning up sessions

[Kropiunig]

Summary

Adds a session-manager plugin with three commands that address the most-requested missing feature in Claude Code: session lifecycle management.

• /session-manager:delete — Delete a session by name, UUID prefix, or interactive numbered selection. Always confirms before deleting.
• /session-manager:list — Cross-project session overview showing timestamps, file sizes, and first-prompt previews — what /resume lacks.
• /session-manager:cleanup — Bulk-remove sessions by age (--older-than=30d) or size (--smaller-than=5KB) with --dry-run support.

Why this matters

Session history accumulates rapidly. Every claude invocation creates a .jsonl file under ~/.claude/projects/, and there is currently no way to delete or review sessions without manually navigating the filesystem. This is the single most duplicated feature request in the repo:

• Need session/conversation deletion and management tools #2562 — "Need session/conversation deletion and management tools" (canonical issue)
• [FEATURE] Add /list-sessions and /delete-session slash commands for better session management #16901 — "Add /list-sessions and /delete-session slash commands"
• Add session management: delete and rename sessions in CLI and VS Code extension #15576 — "Add session management: delete and rename sessions" (closed as dup)
• [FEATURE] Add /delete command to delete current session #25304 — "Add /delete command to delete current session" (closed as dup)

This plugin provides a working solution today while native support is developed.

Design decisions

• Plugin, not core change — Keeps the blast radius small. Users opt in by installing the plugin, and the approach follows the same pattern as commit-commands and other utility plugins in this repo.
• Confirmation-gated deletion — Every destructive operation requires explicit user approval. Cleanup supports --dry-run for preview.
• Companion directory handling — Sessions may have a sibling directory (same UUID) with tool results and subagent data. Both the .jsonl and the directory are cleaned up together.
• Cross-platform — Commands use ~/.claude/projects/ which resolves correctly on macOS, Linux, and Windows (Git Bash / WSL).

Plugin structure

plugins/session-manager/
├── .claude-plugin/
│   └── plugin.json
├── commands/
│   ├── delete.md
│   ├── list.md
│   └── cleanup.md
└── README.md

Follows the standard plugin structure used by commit-commands, code-review, and other plugins in this directory.

Test plan

• Install plugin locally and verify /session-manager:list shows sessions across multiple projects
• Run /session-manager:delete with no args — confirm interactive numbered list appears
• Run /session-manager:delete <uuid-prefix> — confirm single-match direct deletion with confirmation
• Verify deleted session's .jsonl file and companion directory are both removed
• Run /session-manager:cleanup --dry-run --older-than=7d — confirm preview without deletion
• Run /session-manager:cleanup --older-than=7d — confirm deletion after approval
• Test on macOS and Windows (Git Bash) to verify path handling

[Kropiunig]

Bumping this — it's been 4 days since opening with no CI failures (no checks configured on this branch).

This adds a \ plugin with three commands addressing session lifecycle management — the most-requested missing feature. Happy to adjust scope, naming, or implementation approach based on maintainer feedback.