[Feature] Simplify `get` CLI with positional instance and unquoted multi-token query

[mbe24]

Problem

The get command still has avoidable UX friction:

1. instance selection requires -I/--instance, while many users expect positional usage such as 99problems get jira -i 25.
2. -q/--query commonly requires quoting the full query string, which is awkward and makes mixed quote content harder.

This increases command complexity for frequent use and onboarding.

Design

Implement a focused CLI simplification pass:

1. support instance alias as optional first positional argument (99problems get <instance> ...),
2. keep -I/--instance for backward compatibility in this phase,
3. make -q/--query accept multiple unquoted tokens until the next flag and join them with spaces,
4. keep shell quoting semantics unchanged.

Scope

• Add optional positional instance alias to get.
• Preserve -I/--instance support.
• Define conflict behavior when both positional and -I are provided:
  • same value => allowed,
  • different values => usage error.
• Update -q/--query parsing to multi-token capture and token joining.
• Update help/man/README examples for positional instance and unquoted query usage.
• Add parser/regression tests for new CLI behavior.

Boundary

• No removal of -I/--instance in this issue.
• No positional platform support in this issue (positional is instance-only).
• No custom quote interpretation layer beyond normal shell behavior.
• No additional CLI redesign outside instance/query simplification.

Acceptance Criteria

• 99problems get jira -i 25 works as expected.
• 99problems get -I jira -i 25 remains valid.
• If positional instance and -I conflict, command fails with clear usage guidance.
• 99problems get -q is:issue state:open architectural --no-comments parses and executes with full query content.
• Existing non-targeted get behaviors remain unchanged.
• Tests and pedantic clippy checks pass.

Context

This is a small, high-impact CLI ergonomics improvement that keeps compatibility while moving toward lower-friction command usage.

• clap derive docs: https://docs.rs/clap/latest/clap/_derive/_tutorial/index.html
• GitHub CLI UX reference: https://cli.github.com/manual/