refactor: rename agentfs to vfs across runtime, tooling, and docs

[factory-ain3sh]

Description

What

Renames the product surface from agentfs to vfs: workspace crates, CLI binary and command output, validation tooling, benchmarks, and docs. 241 files, net -133 lines, no behavior change.

Why

The runtime stopped being agent-specific; vfs is the name the handoff stack (pack/seed/run PRs above this one) ships under, so the rename lands first to keep those diffs readable.

How

Four mechanical commits, one per surface: workspace runtime, product docs, validation tooling, and the NFS write-handle magic constant.

Four follow-up commits repaired main's pre-existing macOS CI breakage, surfaced once this PR targeted main: 07fea79 and 863e5a7 cfg-gate Linux-only helpers (wait_for_mount, the proc-file writers, report_checkpoint, KnobClass::KillSwitch), 1183037 restores public vfs_mount::unmount and the struct-literal MountOpts idiom in darwin.rs, and 814d725 repairs the .gitignore Icon[ glob and ignores legacy .agentfs/ dirs.

Stack (merged bottom-up 2026-07-28, vfs main @ 3b2af42):

1. refactor: rename agentfs to vfs across runtime, tooling, and docs #4 rename agentfs to vfs <- this PR
2. feat(cli): add vfs pack for atomic session transfer artifacts #5 vfs pack
3. feat(cli): add vfs seed for portable dirty-base capture #6 vfs seed
4. feat(cli): harden resumable run sessions and add status preflight #7 run hardening + status

Related Issue

Part of AC-768

Reviewer Guide

Diff shape: ~95% mechanical identifier/path renames; ~5% docs prose. Skip: benchmark scripts beyond spot checks.
Review depth: Skim. The real check is completeness, not correctness: rg -i agentfs on the branch should only hit historical spec filenames under .agents/specs/.
Read order: commit-by-commit; each commit is one surface.

Risk & Impact

One deliberate break: the NFS write-handle magic changes AFSWRIT\0 -> VFSWRIT\0, so write handles minted by a pre-rename server are invalid after upgrade (clients re-open; no data loss). Everything else is a pure rename; the remaining failure mode is a missed reference in a script or doc, covered by the rg audit above.

Verification

Behavior verified. Workspace builds and the CLI test suite pass at debbd50; rg -i agentfs residuals are confined to dated spec filenames.
Regression coverage. Existing suites unchanged; a missed rename in tooling fails the shell suites directly.
Not tested. N/A, rename only.
Standard validators. cargo build/test and CLI shell suites clean.