feat: inject active mission context into agent system prompt

The session-start hook approach didn't work — Claude Code's TUI
overwrites stdout before the agent sees it, and the hook only fires
when the agent calls it as a tool.

Instead, inject mission context directly into the composed system
prompt via build_runtime_prompt(). When mission.json is active in
CWD, the agent gets mission name, ID, milestone progress, and
mandatory first-action instructions in its initial context.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

bin/mosaic

@@ -169,6 +169,40 @@ EOF
 
   printf '\n\n# Runtime-Specific Contract\n\n'
   cat "$runtime_file"
+
+  # Inject active mission context if present in CWD
+  local mission_file=".mosaic/orchestrator/mission.json"
+  if [[ -f "$mission_file" ]] && command -v jq &>/dev/null; then
+    local status
+    status="$(jq -r '.status // "inactive"' "$mission_file" 2>/dev/null)"
+    if [[ "$status" == "active" || "$status" == "paused" ]]; then
+      local m_name m_id m_count m_completed
+      m_name="$(jq -r '.name // "unnamed"' "$mission_file")"
+      m_id="$(jq -r '.mission_id // ""' "$mission_file")"
+      m_count="$(jq '.milestones | length' "$mission_file")"
+      m_completed="$(jq '[.milestones[] | select(.status == "completed")] | length' "$mission_file")"
+
+      printf '\n\n# Active Mission Context\n\n'
+      cat <<MISSION_EOF
+An active orchestration mission has been detected in this project.
+
+**Mission:** $m_name
+**ID:** $m_id
+**Status:** $status
+**Milestones:** $m_completed / $m_count completed
+
+## MANDATORY First Actions
+
+1. Load \`~/.config/mosaic/guides/ORCHESTRATOR-PROTOCOL.md\` (mission lifecycle protocol)
+2. Read \`docs/MISSION-MANIFEST.md\` for full mission scope, milestones, and success criteria
+3. Read \`docs/scratchpads/${m_id}.md\` for session history, decisions, and corrections
+4. Read \`docs/TASKS.md\` for current task state (what is done, what is next)
+5. Do NOT begin any coding or planning until you have read all four documents above
+
+You are resuming an existing mission. The state files are the source of truth.
+MISSION_EOF
+    fi
+  fi
 }
 
 # Ensure runtime contract is present at the runtime's native config path.