feat: --estimate flag on issues create and issues update

[lightstrike]

Summary

• Add --estimate / -e flag to issues create and issues update commands
• Wire estimate into IssueCreateInput and IssueUpdateInput
• Update no-op guard on issues update to include --estimate
• Add [-e N] to lineark usage for both commands
• Add 3 offline tests for estimate flag visibility and error messages

Test plan

• cargo clippy --workspace -- -D warnings clean
• cargo fmt --check clean
• cargo test --workspace — 67 offline tests pass (3 new)
• Smoke test: lineark issues create "Test" --team ENG --estimate 3
• Smoke test: lineark issues update ENG-xxx --estimate 5

[flipbit03]

@lightstrike column alignment needs to be fixed in some places. usage.rs column alignment is broken. I've had this issue happen so many times in Claude Code-assisted development that I've simply created a linter for it 😁 It should be trivial to fix. Thanks!

[lightstrike]

@flipbit03 Ready for review again — rebased onto main and fixed the column alignment in usage.rs. All CI checks passing.