docs: add Trellis planning and project specs

.trellis/.gitignore

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+# Developer identity (local only)
+.developer
+
+# Current task pointer (each dev works on different task)
+.current-task
+
+# Session/window scoped runtime state
+.runtime/
+
+# Ralph Loop state file
+.ralph-state.json
+
+# Agent runtime files
+.agents/
+.agent-log
+.session-id
+
+# Task directory runtime files
+.plan-log
+
+# Atomic update temp files
+*.tmp
+
+# Update backup directories
+.backup-*
+
+# Conflict resolution temp files
+*.new
+
+# Python cache
+**/__pycache__/
+**/*.pyc