feat(cli): add delete commands for prompts, sessions, and projects

[egdev6]

📋 Pre-flight Checks

• I have searched existing issues and this is not a duplicate
• I understand this issue needs status:approved before a PR can be opened

🔍 Problem Description

Engram's CLI delete surface is still incomplete for administrative cleanup beyond observations.

Observation deletion is already being tracked separately in #209 and currently has work in progress via PR #211. However, there is still no coherent CLI story for deleting prompts, sessions, and eventually whole projects.

That leaves users in an awkward state:

• prompt and session deletes exist through store/server internals, but not as first-class CLI commands
• project cleanup currently relies on engram projects prune for empty projects only
• whole-project deletion would require the lower-level capability proposed in feat(store): add whole-project cascade delete API #214, but there is no tracked CLI follow-up for exposing it once approved

💡 Proposed Solution

Add a consistent CLI family for destructive delete operations beyond observation delete.

Suggested shape:

engram delete prompt <id>
engram delete session <id>
engram projects delete <project>

Notes:

• observation delete is already covered by feat(cli): add engram delete command to remove observations #209 / PR feat(cli): add engram delete command to remove observations #211 and is intentionally OUT OF SCOPE for this issue
• engram projects delete <project> should depend on the project cascade delete capability proposed in feat(store): add whole-project cascade delete API #214
• prompt/session delete commands should reuse the same store/business rules as the existing HTTP endpoints
• destructive commands should print clear confirmation or explicit result summaries
• docs/help output should clearly explain which commands are reversible vs irreversible

Expected behavior:

• prompt delete removes a prompt by ID
• session delete removes an empty session according to existing safety rules
• project delete removes a full project only once the underlying API/store capability exists

📦 Affected Area

CLI (commands, flags)

🔄 Alternatives Considered

• Keeping all delete workflows outside the CLI. That forces users into API calls or direct DB manipulation for administrative cleanup.
• Folding prompts/sessions/projects into feat(cli): add engram delete command to remove observations #209. That would overload an issue/PR already focused on observation delete.
• Waiting for feat(store): add whole-project cascade delete API #214 and not tracking the CLI follow-up. That risks leaving project delete undiscoverable even if the backend capability lands.

📎 Additional Context

Related issues:

• feat(cli): add engram delete command to remove observations #209 — feat(cli): add engram delete command to remove observations (already tracked separately; related PR: feat(cli): add engram delete command to remove observations #211)
• feat(store): add whole-project cascade delete API #214 — feat(store): add whole-project cascade delete API

This issue is intended to track the remaining CLI delete surface so those capabilities are discoverable and consistent once the lower-level primitives are available.