feat(ambient): inject ambient-router skill at session start

[dean0x]

Summary

• Restructures session-start-memory.sh to not early-exit when no memory file exists
• Injects ambient-router/SKILL.md content directly into session context at startup
• Updates ambient-prompt.sh preamble to reference "skill already in your session context" instead of requiring a Read tool call

Problem

ambient-prompt.sh line 38 tells Claude to "Read the ambient-router skill" — this requires Claude to make a Read tool call, which it often skips, making ambient mode appear broken. Additionally, the session-start hook exits early if no .memory/WORKING-MEMORY.md exists, meaning ambient injection would never fire for projects without working memory.

Fix

1. Remove early exit: Session-start hook now builds context incrementally — memory section (if exists) + ambient section (if enabled) — either can fire independently
2. Direct injection: Ambient router skill content is injected into additionalContext at session start, eliminating the need for a Read tool call
3. Simplified preamble: ambient-prompt.sh now references the skill "already in your session context"

Test plan

• npm run build passes (40 skill copies, 20 agent copies)
• npm test — all 178 tests pass
• With memory + ambient: context contains both memory and ambient router content
• Without memory, with ambient: context still contains ambient router content (critical fix)
• Without ambient: no ambient content injected

Closes #79
Part of #78