fix/help: Generate help command list from registered commands

[marcleblanc2]

Part of FE-502. Stacked on #1376 (which is stacked on #1375); retarget to main once those merge. Only the last commit is new here.

Problem

The "The commands are:" block in src help was a hand-maintained string in cmd/src/main.go, and it had drifted from the registered commands:

• src debug (added 2022), src snapshot (2022) and src lsp (2026) were never added to it.
• codeowners was described as "manages code ownership information" in help but "manages ingested code ownership data" in the command itself.
• src doc skipped doc and publish by hard-coded name; publish no longer exists.

Nothing checked that src help, src doc, and the registered commands agreed. #1375 fixed the src doc side of the drift; this PR makes it impossible for the three lists to diverge again.

Change

• New cmd/src/help.go: rootCommands() merges the legacy commands registry and the urfave/cli migratedCommands registry into one sorted list; usageText() renders the help from it. main.go loses the 55-line constant.
• Legacy command struct gains description (one-liner for help) and hidden (excluded from help and from src doc). Every top-level legacy command gets a description; doc is marked hidden and doc.go uses the flag instead of the hard-coded names.
• version (urfave/cli) gets a Usage, so it has a description like every other migrated command. Side effect: version.md from src doc gains that one-line summary (only generated-content change; file set is unchanged at 77).
• Aliases are now shown after the description (orgs manages organizations (alias: org)) rather than in the name column, because batch has five aliases and listing them in the column made it 65 characters wide.

Rendered:

The commands are:

	abc          manages agentic batch changes
	api          interacts with the Sourcegraph GraphQL API
	auth         authentication helper commands
	batch        manages batch changes (alias: batchchange, batch-change, batchchanges, batch-changes, batches)
	code-intel   manages code intelligence data
	codeowners   manages ingested code ownership data (alias: codeowner)
	config       manages global, org, and user settings
	debug        gathers and bundles debug data from a Sourcegraph deployment for troubleshooting
	extsvc       manages external services (alias: external-service)
	login        authenticate to a Sourcegraph instance with your user credentials
	lsp          runs a Language Server Protocol server that proxies requests to Sourcegraph code intelligence
	orgs         manages organizations (alias: org)
	repos        manages repositories (alias: repo)
	search       search for results on Sourcegraph
	search-jobs  manages search jobs (alias: search-job)
	serve-git    serves your local git repositories over HTTP for Sourcegraph to pull (alias: servegit)
	snapshot     manages snapshots of Sourcegraph instance databases (EXPERIMENTAL)
	users        manages users (alias: user)
	version      display and compare the src-cli version against the recommended version for your instance

Tests: help ⇔ registered commands ⇔ generated docs

cmd/src/help_test.go computes the expected command set directly from the two registries (independently of rootCommands()), then:

• TestHelpListsAllRegisteredCommands: the parsed src help list equals that set.
• TestRootCommandsAreWellFormed: every visible command has a description; no duplicate names; no alias collides with a name.
• TestHelpHidesHiddenCommands, TestFormatCommandList.

cmd/src/doc_test.go: TestDocRootIndexMatchesHelp replaces TestDocRootIndexListsAllCommands and asserts the src doc root index.md links exactly the registered set and exactly the src help set (both directions). Together with TestDocGeneratesExpectedFiles (golden 77-file list mirroring OUTPUT_FILES in sourcegraph/sourcegraph) and TestDocLegacyGroupsHaveSubcommandPages from #1375, a new command that is registered but missing from help or from the docs fails CI.

The third leg, "copied into the doc site", is sourcegraph/sourcegraph#15529: _generated.push.sh mirrors doc/cli/references into sourcegraph/docs instead of copying over it, so removed commands disappear from the site instead of lingering.

Mutation checks I ran locally, each caught by the named test:

• drop lsp from rootCommands() → TestHelpListsAllRegisteredCommands and TestDocRootIndexMatchesHelp fail listing lsp
• remove Usage from version → TestRootCommandsAreWellFormed: command "version" has no description
• reintroduce the maps.Copy argument swap → TestDocRootIndexMatchesHelp fails listing all legacy commands

Test plan

• go test ./cmd/src/... passes.
• go run ./cmd/src help renders the list above; go run ./cmd/src version -h shows the new NAME line.
• go run ./cmd/src doc -o /tmp/x vs. the chore: Remove deprecated src teams commands #1376 output: same 77 files, only version.md differs (adds the summary line).