feat(#1787): add /gsd:next smart entry workflow (#1798)

* docs: design spec for /gsd smart-entry command

Hybrid approach porting gsd-pi's smart-entry wizard to gsd-core:
deterministic classifier (gsd-tools smart-entry --json) + markdown
command/workflow with AskUserQuestion + --text fallback. Routing-first
('what now?' menu), 10 situations redesigned for gsd-core's phase loop.

* feat: add /gsd-start smart-entry command

State-aware front door adapted from gsd-pi's smart-entry wizard,
redesigned for gsd-core's markdown-first, multi-runtime architecture.

- src/smart-entry.cts: deterministic situation classifier (no-project,
paused, blocked, verify-failed, needs-first-phase, planning, executing,
verify-pending, idle-stranded, complete, unknown). Reads STATE.md,
ROADMAP.md, git, and verify signals; emits JSON the workflow consumes.
- gsd-tools.cjs: wire case + help listing.
- commands/gsd/start.md + gsd-core/workflows/gsd.md: thin markdown
dispatcher presenting an AskUserQuestion menu (with --text fallback for
non-Claude runtimes) and dispatching to existing commands. Falls back
to /gsd:progress if detection is unavailable.
- help.md: document /gsd:start (parity with bug-2954).
- tests: smart-entry.unit.test.cjs (classifier behavior across all
situations + priority + JSON shape) and gsd-workflow.structure.test.cjs
(markdown-layer invariants + every emitted command resolves to a real
slash command).

Spec: docs/superpowers/specs/2026-06-27-gsd-smart-entry-design.md
Note: command-contract (ADR-0002) requires a gsd:* prefix, so the bare
/gsd from the spec surfaces as /gsd-start.

* refactor: rename smart-entry command to /gsd:next

Rename the command from /gsd:start to /gsd:next per feedback. The
command file is now commands/gsd/next.md (name: gsd:next) and the
backing workflow is gsd-core/workflows/smart-entry.md (named for the
smart-entry classifier and gsd-tools smart-entry subcommand; does not
collide with the existing workflows/next.md, which is the progress
--next sub-workflow). help.md and the spec updated to match.

All affected tests (188) pass; lint:ci clean.

* fix: smart-entry reads real STATE.md schema (nested progress YAML + body Phase field)

Codex review found the classifier misread this repo's own STATE.md: it
looked only for scalar current_phase/total_phases frontmatter and body
fields named 'Current Phase'/'Total Phases', but real STATE.md stores
the phase as body 'Phase: N' and total_phases/percent under a nested
'progress:' YAML object. Both came back null, so active projects
(e.g. this repo at Phase 3 / verifying) wrongly classified as
needs-first-phase.

- detectSignals now reads total_phases + percent from nested progress{}
first, then scalar fm, then body; current_phase falls back to the
body 'Phase:' field (parseProsePhaseField lineage).
- Add regression tests against the real schema (nested progress YAML +
body Phase field) covering verify-pending + executing situations.

Verified against this repo: now classifies verify-pending (was
needs-first-phase). Coverage 93.25% lines / 86.99% branches.

* fix(workflow): tiered fallback when gsd-tools is broken (not just smart-entry)

Live test exposed a self-defeating fallback: when smart-entry --json
failed because gsd-tools itself was broken (missing
markdown-sectionizer.cjs), the workflow fell back to /gsd:progress —
which also depends on gsd-tools and would dead-end too.

Replace the single /gsd:progress fallback with a tiered recovery:
1. Probe gsd_run state-snapshot. If it ALSO errors, the whole tool
layer is down — read .planning/STATE.md directly with the Read tool
and synthesize a minimal situation + actions menu so /gsd:next stays
useful. Surface a rebuild hint.
2. Only if smart-entry alone is missing (older gsd-core), fall back to
/gsd:progress as before.

Matches the direct-read resilience the live agent already did by hand.

* docs: add gsd-next skill surface

* chore: trigger no-mistakes validation

* no-mistakes(review): Fix smart-entry phase ordering

* no-mistakes(review): Fix decimal smart-entry phase ordering

* no-mistakes(test): Fix smart-entry next test contracts

* no-mistakes(document): Docs synced for smart entry

* chore: add changeset fragment for #1798 (/gsd:next smart-entry workflow)