v0.7.0: linkup research subcommand

[shauryajain21]

Summary

Adds a new linkup research subcommand backed by the SDK's async research API. The CLI submits the task, polls until terminal state with a spinner, then renders the result.

New flags

• --reasoning S|M|L|XL — reasoning depth
• --mode answer|auto|investigate|research — research mode
• --output sourcedAnswer|structured — same shape as search, with --schema-file / --schema
• --from / --to (YYYY-MM-DD) — source date range
• --include-domain / --exclude-domain (repeatable) — domain filters
• --no-wait — submit and print task ID without polling
• --id <task-id> — fetch a previously submitted task
• --list — show recent research tasks in a rich table
• Inherits --clipboard, --file, stdin, positional input modes from search

Bumps version to 0.7.0. Updates CHANGELOG and README.

Test plan

• linkup research "capital of Japan" -r S -m answer → completes and returns "Tokyo"
• linkup research --id <id> → fetches completed task
• linkup research --list → shows tasks in a table
• r alias works
• Error paths: bad date format, structured without schema, empty query, invalid JSON schema
• Ctrl+C while polling → prints resume command, exits cleanly