enhance(#4095): checkpoint:decision auto-selection is opt-in via auto_select (#4912)

* enhance(#4095): checkpoint:decision auto-selection is opt-in via auto_select

Auto-mode used to auto-select a checkpoint:decision's first <option>
unconditionally, making a decision checkpoint's safety depend on option
presentation order rather than an authored choice. Add an optional
auto_select="<option-id>" attribute on the <task> tag: absent, auto-mode
now escalates to a human exactly like gate="blocking-human" does; present,
it names the option auto-mode selects; naming an id with no matching
<option id> is a hard structural-validation error at plan-parse time
rather than a silent fallback to the first option. gate="blocking-human"
continues to win over everything, unchanged.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* fix(#4095): anchor auto_select/id attribute regexes past hyphenated decoys

An isolated adversarial review of the auto_select work found that both new
attribute regexes used \b as their left anchor, which is a word boundary,
not a "start of attribute name" boundary. A decoy attribute ending in the
same word (e.g. data-id="...") sitting before the real id="..." on the
same <option> tag matched first, silently corrupting the extracted option
id. Anchor on (?:^|\s) instead so only the real attribute name can match.
Adds a regression test reproducing the exact decoy-attribute shape, plus a
Unicode option-id test from the same review pass.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* test(#4095): register auto-select-attribute.test.cjs in the docs-guard lane

lint-docs-guard-registration failed: the new test reads docs/reference/
plan-md.md but was not registered, so a future edit to that doc could
silently desync from the test without the guard catching it on the PR
that changed the doc. Registered alongside its direct precedents
(precondition-element.test.cjs, reversibility-tagging.test.cjs), which
read the same file for the same reason.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* fix(#4095): fit the decision bullet under execute-phase.md's frozen byte ceiling

The remote gsd-test run caught what local checks missed: execute-phase.md
carries a frozen ADR-857 Phase-6 byte ceiling (93600) with only 36 bytes
of headroom before this change, and the original checkpoint:decision
wording pushed it to 93772 (over the ceiling). Cascaded into failures in
phase6-capstone-conformance, execute-phase-completion-reconciliation,
claude-orchestration, and the compact-content drift-report test.

Also caught: tests/package-legitimacy-gate.test.cjs anchors a
"decision is conditional, not unconditional" safety check on the literal
phrase "first option" in the decision bullet — which #4095 deliberately
removes, since there is no more unconditional first-option pick. The test
was asserting an assumption this change intentionally makes obsolete;
re-anchored on tokens that still identify the bullet ('decision',
'auto-spawn') without weakening what the test actually verifies (the
bullet must still carry a blocking-human carve-out).

Also fixed a word-order mismatch between my own new test's regex and the
actual doc text it was asserting against (tests/auto-select-attribute.test.cjs).

Regenerated the compact-content benchmark baseline
(tests/fixtures/compact-content-benchmark-baseline.json) to match the new
byte counts.

Emitted-Drift-Ack-Growth: gsd-executor.md — +7 bytes (49138 -> 49145), from the auto_select carve-out added to the checkpoint:decision auto-mode bullet; already trimmed once to fit the 49152 hard cap.
Emitted-Drift-Ack-Growth: execute-phase.md — +12 bytes (93564 -> 93576), from the same carve-out in the orchestrator's decision bullet; kept 24 bytes under the frozen 93600 ADR-857 ceiling after two rounds of trimming for clarity vs. margin.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* chore(#4095): backfill changeset PR number

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

---------

Co-authored-by: sim <sim@local>
Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

agents/gsd-executor.md

@@ -331,7 +331,7 @@ For full automation-first patterns, server lifecycle, CLI handling:
 **Auto-mode checkpoint behavior** (when `AUTO_CFG` is `"true"`):
 
 - **checkpoint:human-verify** → Auto-approve **except package-legitimacy checkpoints**. If checkpoint has `gate="blocking-human"` OR its purpose indicates package legitimacy verification (`what-built` mentions `Package verification required before install` or `Package install failed — human verification required`), do **not** auto-approve. STOP and return checkpoint_return_format for explicit human confirmation. Precondition-unmet checkpoints report `blocking-human` — never auto-approved.
-- **checkpoint:decision** → If checkpoint has `gate="blocking-human"`, do **not** auto-select — STOP and return checkpoint_return_format for an explicit human decision (a `blocking-human` decision exists because its default answer would be wrong to assume). Otherwise auto-select first option (planners front-load the recommended choice), log `⚡ Auto-selected: [option name]`, continue to next task.
+- **checkpoint:decision** → If checkpoint has `gate="blocking-human"`, do **not** auto-select — STOP and return checkpoint_return_format for an explicit human decision (a `blocking-human` decision exists because its default answer would be wrong to assume). Otherwise auto-select `auto_select`'s option (absent → STOP like `blocking-human`, #4095), log `⚡ Auto-selected: [option]`, continue to next task.
 - **checkpoint:human-action** → STOP normally. Auth gates cannot be automated — return structured checkpoint message using checkpoint_return_format.
 
 **Standard checkpoint behavior** (when `AUTO_CFG` is not `"true"`):