Docs CLI cleanup: replace stale `nuke` references with `fallout` across docs/

[ChrisonSimtian]

Context

Survey of `docs/**/*.md` shows 112 total NUKE/`nuke` mentions. Many are legitimate (citing NUKE the project, migration framing, attribution) — but a significant subset are stale CLI command references that should now be `fallout`. The codebase itself has already migrated: `src/Fallout.GlobalTool/Fallout.GlobalTool.csproj` declares `fallout`, `Program.Complete.cs` hardcodes `CommandName = "fallout"`, and even this repo's own root-directory marker is `.fallout/` (was `.nuke/`). So this is a docs-content gap, not a code-migration gap.

Top offenders (by CLI-reference count)

`grep -c` of `nuke[[:space:]:]` in `docs/`:

• `docs/01-getting-started/03-execution.md` — 8 references (`nuke [arguments]`, `nuke `, `nuke --help`, `nuke --plan`, etc.)
• `docs/06-global-tool/00-shell-completion.md` — 6 references (`Register-ArgumentCompleter -CommandName nuke ...`, `compctl -K _nuke_zsh_complete nuke`, etc. — completion-script samples need `fallout` as the command name)
• `docs/03-common/06-versioning.md` — 5
• `docs/06-global-tool/03-navigation.md` — 4
• `docs/02-fundamentals/10-logging.md` — 4
• `docs/02-fundamentals/06-parameters.md` — 3
• `docs/06-global-tool/04-cake.md` — 2 (`nuke :cake-convert`, `nuke :cake-clean`)
• `docs/06-global-tool/02-secrets.md` — 2 (`nuke :secrets`)
• `docs/06-global-tool/01-packages.md` — 2
• `docs/01-getting-started/07-telemetry.md` — 2 (`nuke [:setup]`, `nuke :cake-convert`)
• `docs/01-getting-started/02-setup.md` — 2 (`nuke :setup` + `.nuke` root marker — but see "Done when" Enterprise hardening: repo nuget.config breaks corporate packageSourceMapping setups #2)

(Counts exclude lines that are clearly about NUKE-the-project, e.g. `github.com/nuke-build/nuke`, `nuke.build`, `NUKE` capitalized.)

Special cases to handle carefully

1. `.nuke` directory marker in `docs/01-getting-started/02-setup.md` (lines 36, 66) — this is the root-directory marker file, not a CLI command. The repo uses `.fallout` now. Change to match.
2. Shell-completion scripts in `docs/06-global-tool/00-shell-completion.md` need both: `nuke` → `fallout` AND function names (`_nuke_zsh_complete`, `_nuke_bash_complete`) → `_fallout_zsh_complete` etc.
3. Telemetry event names in `docs/01-getting-started/07-telemetry.md` describe events like `BuildSetup` triggered by `nuke [:setup]` — the CLI command in the description should be `fallout :setup`, but verify the actual event names emitted by the runtime haven't been renamed (out-of-scope rename if they have).
4. Don't touch `docs/rebrand-plan.md` and `docs/migration/from-nuke.md` — those are intentionally about the NUKE→Fallout transition and need both names.

Done when

• All CLI command examples in `docs/` use `fallout` instead of `nuke` (excluding the rebrand-plan and migration docs).
• `.nuke` directory marker references in `02-setup.md` updated to `.fallout`.
• Shell-completion samples in `00-shell-completion.md` rebranded (command name + function names).
• Cake conversion docs use `fallout :cake-convert` / `fallout :cake-clean`.
• Secrets docs use `fallout :secrets`.
• Next Falloutdocs deploy confirms no broken-link regressions (`onBrokenLinks: 'throw'` is on).

Refs

• Companion issue: a CLI-bridge guide (alias `nuke` → `fallout` per OS) for users who don't want to retrain muscle memory — that one's about helping people who type `nuke` keep working, this one is about not telling people to type `nuke` in the first place.