ADR 0302: Configured scalar docs join the live path-reference vocabulary

Status: accepted; builds on the paths registry in ADR 0102 and serves the built-in checkpoint set required by the checkpoint decision in ADR 0293.

The shipped gotchas-playbook checkpoint must trigger on the configured gotchas doc — [project].gotchas_doc — in whatever project enables it. The trigger menu already expresses this shape (paths with a live ${<config-key>} reference, expanded against the governing config), but the reference vocabulary did not include the gotchas doc: membership derived solely from the paths registry's configured entries.

Adding the gotchas doc to the paths registry itself is the wrong shape. Registry entries are authored surfaces with a prescriptive non-empty default, and its satellites act on that: setup seeding, the fresh-install scope fills, ownership declarations, and the leakage sentinels all assume a real location. gotchas_doc defaults to the empty string — the feature is off until the owner names a doc — so a registry entry would either lie about a default or force behaviour (seeding, scope membership) the key never had.

The live reference vocabulary has two enumerated sources: the registry's configured members, and a table of configured scalar doc paths.

• SCALAR_CONFIG_PATH_REFERENCES (in src/shared/source_path_references.ts) lists each scalar member: its dotted config key, its ${<config-key>} spelling (registry convention), and a typed resolver. The first and only current member is ${project.gotchas_doc}.
• Expansion treats both sources identically, in every surface that accepts the scope-glob dialect — commands, scope paths and gates, generated paths and runs, checkpoint selectors, standards globs.
• An empty configured value expands to the empty pattern, and the glob dialect defines the empty pattern as matching nothing. A consumer keyed on an unset value therefore goes quiet instead of matching everything — the property that lets a shipped checkpoint reference the gotchas doc safely in projects that never configured one.
• One combined spelling list (LIVE_PATH_REFERENCE_SPELLINGS) feeds the schema descriptions, so the generated config reference documents a new member the moment its table row exists.

• gotchas-playbook can ship with paths = ["${project.gotchas_doc}"]: it tracks the config automatically, activates when the owner names a doc, and never fires while the key is empty.
• The paths registry keeps its meaning — authored surfaces with prescriptive defaults and a seeding story — and its satellites never see scalar members.
• The scalar table is the sanctioned home for future config-pointed docs that need referencing without registry semantics; each addition is one row, guarded by the reference tests and self-documented through the shared spelling list.

• A paths-registry entry with an empty default. Rejected: every registry satellite treats the default as a real location; an empty default would need per-satellite exemptions, spreading the special case instead of containing it.
• Copying the configured doc path into the template entry at setup. Rejected: it duplicates a configured fact, so later moving [project].gotchas_doc would silently strand the checkpoint.