runner: launch bare, and run shell functions as targets

[cameronsjo]

cli-capture required a target on the command line and resolved it through
exec.LookPath, so two things were impossible: starting the tool and deciding
what to run afterwards, and targeting anything that is not a PATH binary. A
zsh function like claude-as failed with a bare "executable file not found".

Two ways in

• No command at all launches an interactive $SHELL as the target.
  ProxyEnv already injects into whatever is launched, so every command run
  inside that shell is captured.
• -shell wraps a named target in $SHELL -ic, which loads the rc file
  first, so aliases, functions, and rc-only PATH entries resolve.

Quoting

Arguments are single-quoted so they stay literal. argv[0] is left bare when
it matches a narrow metacharacter-free charset — in zsh, quoting any part of a
word suppresses alias expansion, so a quoted name would still find a function
while silently missing a user's alias. A security pass confirmed no input after
-- escapes the quoting; the boundary cases are pinned by tests.

-shell is a deliberate waiver of the project's "pass args separately, never
through a shell" rule, with the reason recorded at the call site: resolving a
name only the user's shell knows requires the user's shell, the string is built
from the user's own argv, and the default path stays exec-direct.

Errors

Start now pre-resolves argv[0] and returns ErrTargetNotFound, letting the
command layer name the remedy — -shell for a target the user named, a broken
$SHELL for a derived one.

Before:

start target: exec: "claude-as": executable file not found in $PATH

After:

start target: "claude-as": not found in $PATH — if it is a shell alias or
function, re-run with -shell (or launch a bare shell: cli-capture)

Also

Folds the duplicate POSIX quoting helper in internal/export into a shared
internal/shellquote, so the escaping rule has one home.

Verification

go build ./... && go vet ./... && go test ./... — 16 packages ok, gofmt clean.
Error paths checked against the built binary. The interactive paths (bare launch
lands a shell; traffic from inside it is captured; -shell runs the function)
need a real TTY and were verified only as far as target launch.

Session-Name: bare-launch-shell-target
Session-Id: 011b334d-55e0-41b7-8439-e0ffee421aff
Model: claude-opus-5
Harness: claude-code 2.1.220
Machine: cf6e768835c7

[cameronsjo]

Follow-up commit 5fa9fa3 closes an adjacent gap found during cleanup: the README documented a -leader <key> flag (with a ctrl+space example) that main.go never defined, so passing it exited 2 on an unknown flag.

• -leader is now defined and wins over the config file, mirroring -theme.
• ctrl+space is accepted as a leader name. Terminals send NUL for it — the same byte as Ctrl+@ — so bubbletea reports it as ctrl+@ and the ctrl+a…ctrl+z scan never produced it.

cli-capture -leader ctrl+space -- claude-as alt now reaches the intended shell-alias error instead of a flag-parse failure.