MCP not truly automatic — wire the SessionStart auto-inject hook

[krishpinto]

Problem

query_memory still needs nudging. The strongest fix is the SessionStart hook (engram inject) that drops memories into context before the session starts, with no tool call.

Acceptance criteria

• engram inject --project <slug> produces context output.
• Documented settings.json SessionStart hook.
• Verified: a fresh session has memory context without asking.