feat(cli): generate registry-backed shell completions

[BramVR]

Summary

• add deterministic Bash, Zsh, Fish, and PowerShell completion generation projected from the canonical command registry
• keep Cobra bounded to script generation and hidden completion callbacks; production parsing remains on the standard library
• exclude hidden commands/protocols and avoid config, archive, credential, provider, and file-write effects
• document current-session and persistent installation for all four shells

Proof

• go test ./...
• go test -race ./...
• make fmt-check
• pinned golangci-lint v2.12.2: 0 issues
• make docs-check
• node --test scripts/*.test.mjs
• make docs-site
• govulncheck ./...: no vulnerabilities found
• Windows amd64 cross-build
• built-script byte determinism for all four shells with and without descriptions
• installed-shell parse/load smoke (Bash and Zsh locally; conditional Fish/PowerShell parser coverage in the test suite)
• Codex AutoReview: clean; TruffleHog: clean

Closes #400

[Copilot]

Pull request overview

This PR adds a new gohealthcli completion subcommand that generates deterministic, registry-projected shell completion scripts (Bash, Zsh, Fish, PowerShell) while keeping the CLI’s production parsing on the standard library and using Cobra only for completion script/protocol handling.

Changes:

• Introduces a registry-backed completion command (with --no-descriptions) and wires Cobra-based completion protocol handling ahead of stdlib flag parsing.
• Adds comprehensive tests for script validity, determinism, completion protocol behavior, and “no side effects” (no config/archive/provider reads/writes).
• Updates user documentation and generated command reference pages to include completion setup instructions for all four shells.

Reviewed changes

Copilot reviewed 10 out of 11 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
README.md	Adds completion to the top-level command list.
go.mod	Adds Cobra as a dependency for completion generation/protocol support.
go.sum	Records new dependency checksums (Cobra + transitive deps).
docs/install.md	Documents current-session and persistent completion installation for Bash/Zsh/Fish/PowerShell.
docs/commands/completion.md	New command reference page for gohealthcli completion.
docs/commands.md	Adds completion to the command index.
cmd/gohealthcli/schema_test.go	Updates registry coverage test to include completion.
cmd/gohealthcli/main.go	Routes Cobra completion protocol requests before stdlib parsing/runtime defaults.
cmd/gohealthcli/completion.go	Implements registry-projected Cobra command tree and completion script generation.
cmd/gohealthcli/completion_test.go	Adds determinism/protocol/side-effect tests for completion generation.
cmd/gohealthcli/commands.go	Registers the completion command and wires its Run adapter in init().

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