refactor: clean the core shell and its comments

[ttncode]

What this changes

PR 2 of 5 (spec: docs/superpowers/specs/2026-09-16-clean-shell-and-comments-design.md). Covers scaffold, lib/*.sh, scripts/*.sh and common/install.sh.

• Comments: deletes comments that restate code, narrate history or repeat an ADR. Keeps short why/landmine lines.
• Shell rules: no [ ] tests remain (all are now [[ ]]/(( ))). printf replaces echo wherever an expansion is printed. No local x="$(…)" remains.
• Functions: long functions are split at nameable seams (cmd_add, cmd_wizard, init_project, lint_adapter_env, apply_update_patch, main in install.sh and others). Functions whose names docs cite keep their names and contracts.
• common/install.sh: now set -euo pipefail; it was -uo pipefail. Every step in main is already guarded.
• .git-blame-ignore-revs: added, pointing at the shfmt commit from chore(lint): shellcheck and shfmt every tracked shell file #25.

No user-visible string, exit code or generated file changes. The only exception is bash's own ${1:?} message in adapter-matrix.sh, which now names a different line number because the header got shorter.

How it was verified

• Lint: mise run lint exits 0.
• Tests: mise run test-runner passes 265/265.
• Generated projects: mise run checklist exits 0 in both
  • scaffold new --api flask --web nextjs --db postgres --cache redis
  • scaffold new --api nestjs --db mongodb
• Review: a task review found no behaviour change. Its one finding was a deleted header sentence that tests/workflows.bats cites, restored in a26f8db.

Found and left alone (pre-existing, out of scope):

• set_release_secrets passes the private key via --body, so it is visible in ps.
• pnpm_install runs cd unguarded.
• The found flag in apply_service_dockerfile is dead.

Checklist

• mise run lint passes
• mise run test-runner passes
• New behaviour has a test that fails without the change
• Docs that describe changed behaviour were updated in the same commit
• No unrelated changes