Context layer: refs, aliases, packs, cu onboard, pickers

[timimsms]

Tracking epic for the accepted context-layer design. Full spec: docs/design/context-layer.md (lands via PR #39). Keystone prerequisite: #37.

The four layers

Layer	What it is
Refs	Named resources (support-bugs → list IDs) and saved queries (my-sprint → a compiled Filtered-Team-Tasks call, #25). Consumed anywhere a resource is accepted, via bare name or @name; introspectable via cu ref resolve --json.
Aliases	gh-identical command macros ($1..$9, expand-once, registration-based dispatch). ! shell aliases are legal only from the user's global config or a TOFU-content-hash-trusted project .cu.yml.
Packs	Installable, pinned, declarative-inert git repos bundling refs + aliases + defaults. Trust = a single mechanism: manifest-wide sha256 in the lockfile, semantic-diff re-consent on any change, fail-closed --yes/non-TTY, SHA-pin authority, git fetch hardening, source TOFU.
Interfaces	cu onboard wizard, cu pick, disambiguation prompts, later cu browse — all veneers over the same resolver; nothing interactive is un-scriptable.

Sequencing ladder (S/M/L sizes; details in spec §10)

1. P0 — keystone: config layering rework (config: project .cu.yml merges via viper.Set and Save() writes merged state back to global config #37), remove . from the search path, search/export: unbounded N+1 workspace crawl with silent truncation; adopt Get Filtered Team Tasks #25 Filtered-Team-Tasks rebuild shaped for id slices.
2. P1 — scripting-contract release: universal resolver, resource refs + @ sigil, --json/--jq/--template, cu alias, cu which. The contract freezes here — packs serialize exactly these schemas.
3. P2 — packs: query refs, PATH extensions, internal/ui pickers (bubbletea), cu pick, cu pack (M), cu onboard, dynamic completions.
4. Post-P2 — browse: cu browse [@ref], pack discovery, demand-gated extras.

Maintainer decisions (final; spec §9.1)

1. The wizard is cu onboard, not cu init — onboard reserved now, init future-reserved; no confusion with the existing cu config init.
2. No shell (!) aliases in packs — packs are 100% declarative-inert. Manifest parser rejects ! at install (fail-closed); grant machinery deleted; lockfile = source/pin/rev/manifest_sha256 (+ trusted_projects); cu pack info prints a static capability report; escape hatches = project .cu.yml (PR-reviewed shell macros), cu-<name> PATH extensions, future declarative url: alias type.
3. bubbletea/bubbles in core accepted for internal/ui pickers and, later, cu browse.
4. Future-reserved word list approved (with the init/onboard swap) as an API commitment; cu which --reserved stays.
5. Root cu @ref sugar kept, with a cut line: if @-parsing complicates cobra arg/completion handling, fall back to requiring cu task list @x.

Sub-issues for the individual workstreams will be filed as each phase approaches; none exist yet by design.