Allow axiom: queued only when the signed encoder is blocked

[MaxGhenis]

Follow-up to #9641. That PR was merged on 2026-09-28; a later independent review of its head (95bfb2c) asked for changes, and this PR makes them.

What and why

• queued requires a blocked encoder. CONTRIBUTING.md and CLAUDE.md now say to use queued only when the signed encoder is blocked, to record the blocker in the issue, and that each billed encoder run requires separate approval. Before this, any dispatch-ready issue satisfied parity, even when encoding could go ahead.
• External contributors. The shared guide (PolicyEngine/.github CONTRIBUTING) lets an external contributor write axiom: needed, but the PR template said "Keep exactly one option" of four. The template comment and CLAUDE.md now allow axiom: needed.
• Template checkbox. The YAML-test checkbox applies to policy changes only; otherwise it is marked not applicable.

The review also asked for the #9641 changelog fragment to be in past tense. Towncrier already compiled that fragment into CHANGELOG.md on merge, and CHANGELOG.md is never edited by hand, so this PR's own fragment is in past tense instead.

Companion PRs: PolicyEngine/.github#6 (the shared guide) and PolicyEngine/policyengine-uk#1877.

Test plan

• Documentation and template only; no code or parameters change.
• Changelog fragment in changelog.d/.

axiom: n/a: contributor documentation only

🤖 Generated with Claude Code

[MaxGhenis]

Hand-off to the US + core hub. Head 75c310a.

• Review: an independent hard-tier Subfleet review (job 20261003-222611-axiom-parity-r2) gave VERDICT PolicyEngine/policyengine-us#9783 670e3d6 APPROVE, with no required changes and no nits. Output: ~/PolicyEngine/_drafts/eitc-disqualified-income-2026-09-26/review/guidelines-r2/review_out.md.
• Current head: 75c310a only merges main (f89ded4) to pick up Fix read-only cache tests with Core 3.32.16 #9782's Core 3.32.16 read-only-cache fixture fix. The PR's diff against main is byte-identical to the reviewed diff (git diff cb4e029 670e3d6 = git diff origin/main 75c310a).
• Why the first run failed: on 670e3d6, Full Suite - Rest (Python + variables) failed only test_formulas_do_not_write_into_cached_arrays.py (read_only_put() got an unexpected keyword argument 'derived'). That was main's breakage, which Fix read-only cache tests with Core 3.32.16 #9782 fixed. Nothing in this PR touches code.
• Pending: CI on 75c310a, then an admin merge (--squash --admin --match-head-commit 75c310a…). The only block is REVIEW_REQUIRED, which Max can't give on his own PR.
• Dependencies: Require mirroring policy changes in Axiom RuleSpec .github#6 is merged (a8e190d), so the shared-guide link resolves. The UK companion, Require mirroring policy changes in Axiom RuleSpec policyengine-uk#1877, is merged (8997f3a).
• Holds and decisions: none. Follow-ups owed: none.

axiom: n/a: contributor documentation only

[MaxGhenis]

Merged by the US + core hub (handed off under the hub rule) on the merge gates:

• Head: 75c310a. That is the reviewed head 670e3d6 plus a merge of main (picks up Fix read-only cache tests with Core 3.32.16 #9782). The PR diff is byte-identical.
• CI: 35/35 check runs successful on this head.
• Reviews: an independent hard-tier review approved 670e3d6 (job 20261003-222611-axiom-parity-r2). The hub's GPT-6.1 Sol main-merge check approved 75c310a (~/reviews/us-hub/lander/9783-mainmerge-review.md).
• Scope: contributor documentation only (axiom: n/a).
• Admin merge: used only for the required-review rule, at the reviewed head.