refactor: establish shared foundation for v0.6.0

[takeokunn]

Summary

• pin cl-regex-kit v2.2.0, cl-vcs-kit v0.2.0, and cl-tui-kit v1.0.0 with system-aware sibling derivations
• group package definitions into explicit ASDF modules while preserving serial load order
• establish shared CPS macroexpand and timeout contracts, including 3600s external-command and 300s command-substitution defaults

Verification

• nix flake show --all-systems --no-write-lock-file (exit 0)
• sbcl ASDF load of nshell.asd (exit 0)
• paredit inspect check on all changed Lisp/ASD files (exit 0, 9 files)
• standalone CPS macroexpand contract (exit 0)
• standalone timeout API contract (exit 0)
• full test suite and Nix syntax build were attempted but blocked by a busy shared Nix SQLite database; full CI remains authoritative

Scope

Wave 0 only: baseline, dependency pins, ASDF modules, and shared CPS/test/timeout foundation. No merge or release performed.

[takeokunn]

Independent review found blocking gaps; do not merge yet.\n\n1. PTY coverage is skipped in CI: t/helpers-runner.lisp:60-70 makes skip-when-pty-round-trip-unreliable skip whenever CI is set, and t/integration/test-pty.lisp:386-406 places the important pty-open-write-read-close round-trip behind that macro. The observed CI result is 2329 selected, 2328 passed, 1 skipped, so a PTY regression can pass all required jobs. Please provide a deterministic CI PTY verification path (or an equivalent explicitly gated job) and make the gate fail when the required PTY check is unavailable.\n\n2. Timeout predicate mutation survives: src/infrastructure/acl/timeout.lisp:9-13 is covered only by truthy default checks at t/unit/test-execute-pipeline.lisp:443-447 and 606-610. Replacing timeout-seconds-p with a constant true result leaves the suite green; negative/non-numeric/non-finite inputs are not asserted. Add boundary contract tests and reject non-finite values if the predicate promises finite timeouts.\n\n3. The launch acceptance condition requires removing macOS/ARM system evaluation, but flake.nix:176-179 still declares aarch64-darwin. CI has no macOS job, but the flake still evaluates that platform. Remove it or explain and update the acceptance condition before merge.\n\nCI evidence: all four Ubuntu x86_64 checks pass, but the skipped PTY test means this is not sufficient evidence for merge.