Feature: non-watch snapshot for `sentry local` (`local dump` / `serve --once`)

[sergical]

Summary

sentry local is watch-only today: serve tails a live SSE stream and run wraps a process. There's no non-interactive way to grab the events captured so far and exit. A one-shot sentry local dump (or a --once flag on serve) would print the current buffer and return, which is handy for scripts, CI checks, and AI coding agents.

Motivation

The server already retains a buffer of recent envelopes for late subscribers, but the only way to read it is to attach to the stream and disconnect manually (serve -F json | head, or curl -N localhost:8969/stream). That's awkward for:

• Agents/automation that want "what did my app emit in the last run?" as a single command with a clean exit code.
• CI / scripts asserting on captured events without managing a long-lived process.
• Quick inspection without entering watch mode.

(Related but distinct: #998 adds --verify — run app, wait for first envelope, pass/fail, exit — and #809 is synthetic event generation. Neither gives a snapshot of captured events.)

Proposed behavior

• sentry local dump — connect to a running server (or the buffer), print everything currently buffered, then exit 0. Honors the existing -f/--filter (error|transaction|log|ai) and -F/--format (human|json) flags.
• Or equivalently sentry local serve --once / --exit-after-drain: replay the buffer and exit instead of streaming.
• Errors with a clear message if no server is running (or starts an ephemeral one for the run-style case — TBD).

# Snapshot the buffer as NDJSON and query it
sentry local dump -F json | jq 'select(.type=="error")'

# Just the AI/agent spans captured so far
sentry local dump -f ai

Notes / open questions

• Scope is the in-memory Spotlight buffer (last N envelopes), not a persistent store — same data serve already replays to late subscribers.
• Should dump auto-start a server if none is running, or only read an existing one? (Probably read-only; error if none.)
• Naming: dump vs serve --once vs tail --once.

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