v0.3.0: uvx install, SKILL.md, session import

Three real gaps closed, not just planned

uvx: nothing to pre-install. Every client's config example now shows
uvx --from 'spomory[llm,embedding,mcp]' spomory-mcp as the recommended
way to run Spomory -- no pip install, no virtual environment to build
by hand, and it sidesteps the macOS ~/Documents TCC gotcha for free
(uv's cache was never under there). The manual venv install is still
documented as an alternative for anyone who wants a pinned version.

skill/spomory/SKILL.md: the tools are reactive, this makes them get
used. The six memory tools don't call themselves -- an agent has to
decide to. This skill teaches "recall first, store last": search memory
before assuming you don't know something, save durable facts as they
come up without being asked, and (for a genuinely empty account) run a
short onboarding flow instead of starting cold. Copy the spomory folder
into your client's skills directory (.claude/skills/, .cursor/skills/,
etc.) -- open SKILL.md format, works anywhere that supports it.

spomory-import-sessions: fix the empty-on-day-one problem. A new
console script that reads Claude Code's, Codex's, or Cursor's own local
session history and runs it through the exact same extraction pipeline
add_memory uses, so a fresh account starts with real memories instead
of nothing:

spomory-import-sessions --client claude-code   # or codex / cursor

Parsers were written against real session file formats read from a real
machine, not guessed from a spec. This command has real side effects --
it reads your actual local conversation content and sends it to whichever
LLM you've configured -- so it's documented, not silently automatic.

Also in this release

• The REST API (/v1/memories, /v1/graph, /v1/export) shipped
  earlier now has properly declared OpenAPI securitySchemes, so tools
  that auto-import the spec (n8n, Zapier, ChatGPT Actions) correctly
  prompt for a credential instead of silently 401ing.
• A real pre-existing bug fixed: export_memory 500'd on any relation
  whose provenance had session_id=None (the common case) -- the export
  schema's provenance field was still typed for a version before
  client_name/session_id existed.