refactor(secrets): store lives at /secrets, not /srv/secrets

/srv is the projects tree — agents grep, find and `ls -R` it constantly, so a store
under it turns up in ordinary searches and risks being read (or pasted) by accident.
/secrets sits outside that blast radius: nothing routinely walks it, and it is still
0700 loops, still not a repo, still ciphertext at rest.

Override with AO_SECRETS_STORE if a host puts it elsewhere.

Co-Authored-By: Claude <noreply@anthropic.com>

README.md

@@ -363,11 +363,11 @@ Run it by hand with `engine/agents.py up --config agents.toml`.
 secret anywhere else** — not in a git remote URL, not in a project `.env`, not in a prompt.
 
 ```
-/srv/secrets/store.yaml         the store: sops+age ciphertext, mode 0600
+/secrets/store.yaml         the store: sops+age ciphertext, mode 0600
 ~/.config/sops/age/keys.txt     the age private key — the ONE plaintext secret, mode 0600
 ```
 
-`/srv/secrets/` is deliberately **not a git repo and has no remote**, so there is no path by
+`/secrets/` is deliberately **not a git repo and has no remote**, so there is no path by
 which a `git add`/`git push` can leak it; the store is ciphertext at rest anyway.
 
 Read it with `engine/secrets.py` (stdlib + the `sops` binary, no Python deps):
@@ -382,7 +382,7 @@ env    = get_group("cc_ci_testenv")       # a whole group as a dict
 python3 engine/secrets.py list                 # group/key NAMES only — never prints values
 python3 engine/secrets.py get tangled.cookie   # one value on stdout
 python3 engine/secrets.py materialize tangled-session   # write a runtime file from the store
-sops /srv/secrets/store.yaml                   # add/edit: decrypts to $EDITOR, re-encrypts on save
+sops /secrets/store.yaml                   # add/edit: decrypts to $EDITOR, re-encrypts on save
 ```
 
 **Materialized files.** Some consumers read a fixed path and can't be taught otherwise (a systemd