feat: add fish shell tab completions

[nitishagar]

Issue for this PR

Closes #1515

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds fish shell tab completion support. Bash/zsh completions already work via yargs' built-in .completion() method, but fish uses a completely different format (complete -c opencode ...) that yargs can't generate.

Changes:

• New packages/opencode/src/cli/cmd/completion.ts with a generateFishCompletions() function that outputs a static fish completion script covering all commands, subcommands, options, and choices
• Pre-parse interception in index.ts: when opencode completion --shell fish is invoked, the fish script is output before yargs processes the args. Existing bash/zsh behavior is completely untouched
• Updated nix/opencode.nix to install fish completions alongside bash/zsh

Usage:

opencode completion              # bash/zsh (unchanged)
opencode completion --shell fish # fish (new)

The fish script uses standard fish patterns (__fish_seen_subcommand_from, set -l subcommand lists) to gate completions properly for nested subcommands like opencode debug lsp diagnostics.

How did you verify your code works?

• Ran bun run typecheck — no errors in changed files
• Verified fish output via bun -e "import { generateFishCompletions } from './packages/opencode/src/cli/cmd/completion.ts'; console.log(generateFishCompletions())" — produces 237 lines of valid fish completion syntax
• Verified existing .completion() call and --get-yargs-completions mechanism are untouched

Screenshots / recordings

N/A — CLI feature, no UI changes.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[nitishagar]

CI check-duplicates timed out on first run — not related to code changes. Requesting re-run.