docs(openspec): remove the stale AGENTS_FALLBACK requirement

[theCodeDrift]

Removes the cli-init requirement "Codex install destination overrides the fallback for the same directory". This is a spec-only change, with no code or release note.

What it claimed

• An AGENTS_FALLBACK tool descriptor existed alongside Codex, both with installDir: ".agents".
• A "state-based cleanup helper" looked up tool descriptors by installDir and had to return Codex rather than AGENTS_FALLBACK.
• The install summary named "Codex" for .agents/skills/ writes when .codex/ was present.

Evidence it is stale

• grep -rn "AGENTS_FALLBACK" packages/cli/src finds nothing.
• TOOLS in packages/cli/src/install/install.ts is detection-only. The install destination comes from the fixed SHIM_TARGETS catalog, which has a single .agents row (label: "Agent Skills", generic: true). DEFAULT_SHIM_DIR = ".agents" is what detectSelectedDirectories returns when nothing is detected.
• packages/cli/src/install/state.ts has no tool-descriptor lookup at all. It diffs manifest targets by directory name.
• The summary-labelling scenario contradicts "Wizard prompts the user to choose install locations", which says the .agents/ target SHALL be reported in the install summary as Agent Skills. The code follows that requirement.

Was any scenario still live?

None is lost:

Removed scenario	Status
Codex detection labels the .agents/ install as Codex	Contradicted by the wizard requirement. Codex is now named on the picker row, which "Codex is named in the tool list" specifies
Lookup by installDir resolves to Codex over fallback	The helper and descriptor do not exist
Fallback still resolvable for legacy state without Codex detection	Covered by "Agents fallback install" (".agents stub written when no tools detected") and by the wizard's "Agents is the default when nothing is detected or installed" and "Manifest-recorded locations are pre-checked"

Scenario diff

I diffed the sorted #### Scenario lines of openspec/specs/cli-init/spec.md before and after openspec archive. Only the three scenarios above disappeared, and nothing was added or changed:

< #### Scenario: Codex detection labels the .agents/ install as Codex
< #### Scenario: Fallback still resolvable for legacy state without Codex detection
< #### Scenario: Lookup by installDir resolves to Codex over fallback

The change is archived on this PR as openspec/changes/archive/2026-10-15-remove-agents-fallback-requirement/. pnpm openspec validate --strict passed before archiving.