docs: document tab completion and bring the README up to date

[iamteedoh]

What does this PR do?

Three releases of behavior landed (#7, #9, #10) and parts of the README still described the tool as it was before. This audits the whole file against what v0.3.0 actually does.

Documents tab completion

The prompt has completed paths ever since readline arrived alongside the arrow-key history, and nothing said so — which means nobody would ever try it. Adds a keys table under Interactive: Tab, ↑/↓, Ctrl-A/E/W/U/R, q.

The table records what was verified over a pty, not what readline is assumed to do. Two claims were corrected during testing:

• An earlier draft said "press Tab twice to list the candidates". That is false for read -e — it fills the common prefix and beeps without listing. Re-tested on a 120×40 pty in case readline was suppressing the list on a 0×0 terminal; it was not.
• The session's history never touches your shell history (HISTFILE is cleared), so that is now stated rather than left to guesswork.

Corrects what had gone stale

Was	Now
Overview: prompts "for the input file and the permission to check"	a session taking a path or a list file, looping until q
Feature bullet advertising owner rwx | group r-x | other r--	that format no longer exists — replaced with the Mode column
"Interactive prompts guide the user to select the input file"	predated --path
"prompts for the rest when a terminal is attached"	had drifted under the Paths vs. lists heading during earlier edits, reading as though it were about disambiguation — moved back
Prerequisites	now name the Bash 3.2 floor (what macOS ships), the tty the session needs, and that getent/dscl are only for another user's ~
Installation: "save the script content to a file"	predated the repo having releases — clone or download the release

Adds an Exit codes section

The README recommends the tool for cron and CI but never documented that a denied permission exits 0 — only usage errors exit 2. Anyone writing dirPathPerms.sh … || alert would get silence forever. Now documented, with a grep-based recipe for acting on the result.

Related issue

DIR-5

Validation

Documentation-only, so the value is in whether the claims are true. Every runnable claim was executed rather than eyeballed:

• All 16 documented flags exercised (-P, --path, -f, --file, -w × 4, -p × 5, -a/--all, --no-color, -h, -V, positional, repeatable -P) — all pass.
• Exit codes verified against the new table: denied → 0; missing list file, unknown option, --file+--path, and no-tty-no-path → 2.
• The CI recipe was run: it fires on a group-writable directory and stays silent when nothing matches.
• Tab completion driven over a pty for /etc/pass, /var/lo, ~/Doc.
• All README internal anchors resolve.
• Repo gate unchanged and green: shellcheck, bash -n, 47/47 tests, gitleaks.

Notes for review

Two things deliberately not done here:

• Symlink caveat. Verifying a README example surfaced a real bug — /tmp reports the symlink's mode (lrwxr-xr-x) instead of the target's (drwxrwxrwt), so the tool answers "not group-writable" when it is. Raised as DIR-6. Documenting it as a caveat would enshrine a bug that should be fixed instead; say the word if you'd rather ship the caveat.
• The hero image in assets/ still shows v0.1.0 and the old File path: prompt — the exact UX feat: check paths directly, with tilde/variable expansion and prompt history #7 removed. It needs re-capturing from a real terminal; there is no terminal-to-image tooling available to regenerate it here.

docs: — README-only, so release-please will not bump the version for it.

Checklist

• git ls-files '*.sh' | xargs shellcheck passes
• git ls-files '*.sh' | xargs -n1 bash -n passes
• gitleaks reports no secrets in Git history
• Scripts and programs include a GPL-3.0-or-later SPDX header
• No secrets, tokens, credentials, or private infrastructure details are committed
• Documentation is updated for user-visible or operational changes
• The PR title follows Conventional Commits