feat(jtk): INT-693 clear fields with an empty value; none/null clear numbers

[piekstra]

[INT-693]

Signed replacement of #470 — that branch's commits were unsigned and the main ruleset requires verified signatures. This commit was created via GitHub's API (GitHub-signed, verified) and its tree SHA (cd407189) is identical to #470's reviewed head, so the content is byte-for-byte what cr APPROVED there with all CI green and its one finding (issuelink clear-on-empty test) addressed.

jtk issues update -f 'Story Points=' (or =none) previously failed with Jira's per-type validation error because FormatFieldValue only honored null-intent for user fields; there was no way to unset a custom field from the CLI.

The contract

• Empty value clears every structured type (number, option, array, priority-like, issuelink, parent) → JSON null
• none/null additionally clear number fields, matching user-field behavior
• Option-like fields do NOT treat None as a clear — a select list may contain a literal "None"
• Free-text unchanged: verbatim value; empty string is the idiomatic text clear

Validation

TestFormatFieldValue_Clearing covers every clear branch incl. issuelink; full tools/jtk suite green; gofmt clean.

[monit-reviewer]

Automated PR Review

Reviewed commit: 43cf2f65e2f2
Profile: claude-monit-reviewer - Posting as: monit-reviewer

Summary

Reviewer	Findings
go:implementation-tests	1
policies:conventions	0

go:implementation-tests (1 finding)

Minor - tools/jtk/api/fields_test.go:950

TestFormatFieldValue_Clearing tests that the option branch keeps a literal "None" value (since a select list may legitimately have an option named "None"), but the priority/resolution/status/issuetype/securitylevel branch — which shares the exact same name-addressed, non-none-clearing contract per the new doc comment — has no equivalent case. This is the one branch in the group where the untested edge is realistic rather than hypothetical: Jira ships a built-in Priority literally named "None" in many instances, so --field priority=None hitting this code path is a real, not speculative, scenario, and it's exactly the case the PR's contract calls out ("a select list or priority scheme may legitimately contain an entry named 'None'"). Add a case mirroring "option keeps the literal string None" for a priority-typed field (e.g. {name: "priority keeps the literal string None", field: priorityField, value: "None", want: map[string]string{"name": "None"}}) to lock in the behavior the doc comment promises.

Reviewer Coverage

• go:implementation-tests — complete (broad); skipped: none; constraints: none
• policies:conventions — complete (broad); inspected 2 assigned files (3 inspected across reviewers): tools/jtk/api/fields.go, tools/jtk/internal/cmd/issues/update.go; skipped: none; constraints: none

Inspected files (3)

• tools/jtk/api/fields.go
• tools/jtk/api/fields_test.go
• tools/jtk/internal/cmd/issues/update.go

0 PR discussion threads considered. 0 summarized; 0 resolved.

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Completed in 2m 19s | ~$1.79 (est.) | claude-sonnet-5 | cr 0.10.282

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Reviewers	go:implementation-tests, policies:conventions
Engine	claude_cli · claude-sonnet-5
Reviewed by	cr · monit-reviewer
Duration	2m 19s wall · 3m 07s compute
Cost	~$1.79 (est.)
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