fix: correct alias matching, portable prompts, and exit codes

[Jintin]

Summary

Fixes several verified correctness bugs in aliasme.sh, found during a code review pass. All fixes were verified by reproduction in both bash and zsh; the existing test suite passes.

Wrong-command execution / data corruption (pair-wise matching)

_excute and _remove compared every line of ~/.aliasme/cmd against the given name — including command lines. With aliases x -> deploy and deploy -> echo deploying:

• al rm deploy matched x's command line, deleted the wrong pair, and left the file corrupted so al x silently ran the wrong command.
• Similarly, execution could match a command line and eval a name line.

Both now step through the file in name/command pairs (like _list already did), via a new _find helper.

Interactive prompts broken (both shells)

• al rm with no argument used read -pr, which errors in bash (not a valid identifier) and zsh (no coprocess).
• al add with no arguments used read -rep, a bashism that fails under zsh (-p: no coprocess), storing aliases with empty names.

Replaced with portable printf + read -r.

Exit codes

al previously returned 0 on every path. Now:

• al <unknown> prints not found: <name> and returns 1
• al <name> propagates the executed command's exit status, so al foo && ... chains work

Store hygiene

• al add rejects duplicate names (previously appended silently; execution used the first entry while ls showed both) and empty names/commands
• All function variables are local, so sourcing the script no longer clobbers $name, $cmd, $line, $opts in the user's interactive shell
• IFS= on all read loops so whitespace round-trips

Test plan

• test/aliastest.sh passes (4/4)
• Manual verification in bash and zsh: pair-collision execute/remove scenarios, interactive add/rm via stdin, duplicate rejection, exit-code propagation

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