feat(session): add CLI session workflows

[KurosawaGeeker]

Summary

• Add a recall session CLI group for agent-friendly session workflows: list, show, export, share, resume, and open.
• Move the session subcommand implementation into src/session.rs so src/main.rs remains focused on top-level routing and existing command flows.
• Add structured JSON/JSONL output paths so coding agents can select a session and act on it without driving the TUI.
• Extend export::ExportOptions with explicit session_ids, so session export --format jsonl reuses the existing JSONL export writer instead of bypassing export logic.
• Share the DB listing query path between indexed session listing and bulk export filters to reduce duplicate SQL.
• Tighten Cloudflare Pages share setup by generating readable project names with a short random suffix, such as recall-share-7f3a2c, and resolving the actual Pages domain from Wrangler project metadata instead of guessing project_name.pages.dev.
• Document the proposed CLI behavior in docs/session-cli-prd.md and add README usage examples.

Validation

• cargo fmt
• cargo check
• cargo clippy --all-targets -- -D warnings
• cargo test
• Manual CLI checks for session list, session show, session export, session share --dry-run, session resume, and session open.
• Manual check that recall record is not exposed.
• Manual real Cloudflare Pages share for one Codex session returned a public URL and curl -I returned HTTP/2 200.

Release notes

• No generated files included.
• No GraphQL codegen updates.
• No DB baseline updates.