Store Windows agy tokens in Credential Manager for profile switch

[Ishannaik]

Why

On Windows, agy.exe reads the live Antigravity token from Credential Manager target gemini:antigravity at process start. Copying ~/.gemini files does not change that slot, so switch and login left the live Windows login in place.

What

ctypes/advapi32 Generic credentials (CRED_TYPE_GENERIC, 2560-byte blob cap):

• Live slot: gemini:antigravity (override with AGY_WINCRED_TARGET)
• Saved profiles: agy-cli-manager:<name>
• switch / activate copy a profile blob onto the live target via apply_to_live
• login snapshots the live slot, lets agy write a new blob, then copies it to agy-cli-manager:<storage>; restore the snapshot if login fails
• Capture/activate of the real live slot refuses while agy.exe or Antigravity* is running; --force overrides
• One-shot migrate: wincred_antigravity.json -> profile target + credential.meta.json (sha256/length only), then delete the JSON
• Tests write only agy-cli-manager-test:*; they never touch gemini:antigravity

Windows Credential Manager  CRED_TYPE_GENERIC
┌──────────────────────────────────────────────────────────────┐
│  gemini:antigravity           live slot agy.exe reads        │
│  agy-cli-manager:<profile>    saved standby blobs            │
│  agy-cli-manager:backup:live  login-time snapshot of live    │
└──────────────────────────────────────────────────────────────┘

switch <name>
  agy-cli-manager:<name>  --apply_to_live-->  gemini:antigravity

login
  gemini:antigravity --> agy-cli-manager:backup:live
  agy CredWriteW     --> gemini:antigravity
  gemini:antigravity --> agy-cli-manager:<storage>
  success: delete backup    failure: restore backup then delete

legacy
  accounts/<name>/wincred_antigravity.json
       --> CredWriteW(agy-cli-manager:<name>)
       --> credential.meta.json (hash/length; no blob)
       --> unlink JSON

flowchart LR
  subgraph credman [Windows CredMan Generic]
    Live["gemini:antigravity"]
    Prof["agy-cli-manager:<profile>"]
    Backup["agy-cli-manager:backup:live"]
  end

  agy["agy.exe start"] -->|CredReadW| Live
  Prof -->|"switch / apply_to_live"| Live
  Live -->|"login snapshot"| Backup
  Backup -->|login fail restore| Live
  Live -->|"login capture"| Prof
  JSON["wincred_antigravity.json"] -->|migrate_legacy_json then unlink| Prof
  JSON --> Meta["credential.meta.json"]

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Usage

agy-cli-manager login work
agy-cli-manager switch work
agy-cli-manager switch work --force

--force is the live-slot process-guard override. After a switch, restart agy; it still holds the old token in memory.

Tests

PYTHONPATH=src pytest tests/test_wincred.py tests/test_watch.py: 26 passed