feat(cli): shell-init eval hook, prompt segment & completions (spec 30 phase 2)

[gustavobertoi]

Summary

Phase 2 of the interactive-DX lane (spec 30): the shell integration that makes devstack use switch your live shell — the "execute, don't print" fix. Builds on phase 1 (#115, merged).

What's in

• devstack shell-init zsh|bash|fish — prints shell code to eval from your rc: install dir on PATH, a devstack() wrapper that eval's use's output, completion loading, and a devstack_prompt helper. Named after the invoked binary, so an alias (rq) gets an rq() wrapper.
• use --print --shell <shell> — emits POSIX (sh/zsh/bash) or fish syntax; bare use --print sends its hint to stderr so stdout stays an eval-safe (empty) script.
• context --prompt — a cheap prompt segment (config + DEVSTACK_PROJECT only; no Docker/ledger): prints workspace or workspace:project.
• install.sh — detects the shell and prints the exact eval line (never edits your rc).

Verified end-to-end

$ eval "$(devstack shell-init bash)"
$ devstack use web        # wrapper eval's cd + export DEVSTACK_PROJECT=web
$ echo $DEVSTACK_PROJECT  # web  ← the live shell changed
$ devstack context --prompt   # smoke:web

Tests / gates

New shell_init_test.go (per-shell script gen, alias wrapper, unknown-shell error, context --prompt). make ci (fmt/vet/build/test-race) + make determinism green; sh -n install.sh clean.

[Q-PROMPT-COST] addressed (prompt path skips Docker/ledger). Next: Phase 3 — use fuzzy-picker TUI, project TUI, env/secrets TUI.

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