feat: show estimate in issues list, search, and read output

[lightstrike]

Summary

• Add estimate: Option<f64> to IssueSummary, SearchSummary, and IssueDetail types
• Add estimate column to IssueRow table output
• Format estimate as integer when whole number (e.g. "3" not "3.0"), empty string when None
• Estimate now appears in issues list, issues search, and issues read output

Test plan

• cargo clippy --workspace -- -D warnings clean
• cargo fmt --check clean
• cargo test --workspace — all 64 offline tests pass
• Smoke test: lineark issues list --limit 5 — estimate column visible
• Smoke test: lineark issues search "test" -l 5 — estimate column visible
• Smoke test: lineark issues read ENG-xxx — estimate field in detail output

[flipbit03]

LGTM, from a functionality standpoint, but we should not delete these comments from unrelated pieces. Issue documented in #97

[lightstrike]

@flipbit03 Ready for review again — rebased onto main and reverted the unrelated comment deletions. All CI checks passing.