feat(complete): complete sigil-prefixed positionals

[jdx]

Summary

• match completions against the value after the declared sigil
• restore the sigil on emitted completion candidates
• keep explicit double dash behavior intact
• add dedicated sigil documentation and completion examples

Stack

• Depends on feat(spec,parse): add sigil-classified positional arguments #1322

Test plan

• cargo test -p usage-cli complete_word_sigil
• cargo test -p usage-conformance --test sigil
• mise run lint

AI-assisted — Tool: Codex; model: OpenAI/GPT-5; version: unavailable.

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Note

Low Risk
Changes are confined to completion paths with new tests; parsing behavior is assumed from the dependent stack (#1322), not modified here.

Overview
Adds shell completion for sigil arguments (arg sigil="+" and similar): tokens like +n are treated as the declared positional after stripping the prefix, then candidates are emitted with the sigil restored (+node@22, etc.).

Both usage-argv (complete.rs) and the CLI reference (complete_word) share the same rules: longest matching sigil wins, flag values and explicit -- / restart boundaries / double_dash="automatic" stop sigil classification, and cwd file fallback is suppressed while completing a sigil-shaped word (including dynamic run= completers that see the stripped words[CURRENT]).

Documentation adds a sigils reference page, a sigil.usage.kdl example, and tests cover choices, separators, restart tokens, overlays, and runtime completers.

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[github-actions]

Instruction counts

benchmark	trend	instructions	Δ	wall (min)	Δ
markdown	▅▁▁▄▅▁▁██	338,970,876 → 339,060,552	+0.03%	30.04 → 29.71ms	-1.10%
startup	▂▂▂▆▆▆▆█▁	911,202 → 907,830	-0.37%	0.91 → 0.94ms	+2.71%

No instruction-count regression above 1%.

_{Only instruction counts gate. Wall clock is shown for context — on identical hardware it moves 4-20% run to run.}

_{Measured by tak — instruction-counted CLI benchmarks, stored in this repository's git notes.}

Shadow comparison

Parsing mise use -g node@20 against a shadow of mise's committed spec.
Reported, not gated: the shadow grows as the derive learns to express more, so
what to watch is the ratio rather than either column.

framework	stripped binary, bytes
usage	1328960
bpaf	2493280
clap	3101832

framework	instructions, cold parse	vs usage
usage	8477	—
clap	6315570	745x
bpaf	21909031	2584x

                                              min       p01       p10    median
usage-rs: argv -> struct                      419       423       426       440  ns
clap: build tree + parse -> struct         526025    526588    529664    549417  ns
bpaf: build parser + parse -> struct      1581456   1581456   1591780   1607436  ns

usage: argv -> struct                             422 ns      0.42 µs
clap: build tree + parse -> struct             532204 ns    532.20 µs
clap: parse -> struct, tree reused              23474 ns     23.47 µs
clap: build tree only                          324053 ns    324.05 µs

_{895f5f3cf949 vs 5c9ddcf0fac0 · measured on the runner, not pushed to the history.}

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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