Make the Vault gitea creds path selectable

agentpr always read gitea/creds/unkin-agent from a bare const, so a service
like repospawner could not run it as its own Gitea identity.

- Replace the GiteaCredsPath const with a function: GITEA_CREDS_PATH when set,
  otherwise gitea/creds/<AGENT_LOGIN>. Unset env still resolves to
  gitea/creds/unkin-agent, so existing callers are unchanged.
- Thread the creds path through fetchGiteaToken/readGiteaCreds instead of
  reading a package-level const, and report it in the error messages.
- Make agentpr's help text login-agnostic and document both variables.

AGENTS.md

@@ -3,8 +3,10 @@
 ## Project Overview
 
 This repo ships several Gitea-automation CLIs in one RPM (`agent-tools`). They
-act as the `unkin-agent` user by minting a scoped Gitea token from Vault, so
-actions are attributed to the agent rather than to whoever runs the tool.
+act as an agent user (`unkin-agent` by default) by minting a scoped Gitea token
+from Vault, so actions are attributed to the agent rather than to whoever runs
+the tool. Setting `AGENT_LOGIN` selects a different agent identity, so a service
+like repospawner can run these tools as itself.
 
 - **`agentpr`** — create pull requests and post PR comments as `unkin-agent`
   (fixes the "tea posts as Ben" attribution problem). Subcommands:
@@ -32,7 +34,7 @@ cmd/agentws/main.go      # agentws CLI (new / list / rm / clean / token / creden
 cmd/agentvault/main.go   # agentvault CLI (seed-outpost)
 internal/agent/          # shared plumbing:
   token.go               #   env config + in-process Gitea-token cache
-  vault.go               #   AppRole login + read gitea/creds/unkin-agent
+  vault.go               #   AppRole login + read the gitea creds path
   gitea.go               #   Gitea REST client (PR create/get, comments, status, whoami)
   parse.go               #   owner/repo#N and owner/repo parsing
   watch.go               #   PRState snapshot + MeaningfulChange comparison
@@ -57,7 +59,9 @@ All tools call `agent.GiteaToken()`, which (once per process):
 
 1. AppRole login: `POST $VAULT_ADDR/v1/auth/approle/login` with `role_id` only
    (no `secret_id`) → `client_token`.
-2. `GET $VAULT_ADDR/v1/gitea/creds/unkin-agent` with `X-Vault-Token` → `.data.token`.
+2. `GET $VAULT_ADDR/v1/<creds path>` with `X-Vault-Token` → `.data.token`, where
+   the creds path is `GITEA_CREDS_PATH` if set, else `gitea/creds/$AGENT_LOGIN`
+   (so unset env still reads `gitea/creds/unkin-agent`).
 
 Config via env (all have defaults):
 
@@ -66,7 +70,8 @@ Config via env (all have defaults):
 | `VAULT_ADDR` | `https://vault.service.consul:8200` | Vault/OpenBao address |
 | `AGENT_APPROLE_ROLE_ID` | built-in default | AppRole role_id (overridable) |
 | `GITEA_URL` | `https://git.unkin.net` | Gitea base URL |
-| `AGENT_LOGIN` | `unkin-agent` | login whose comments watchpr ignores; agentws git identity |
+| `AGENT_LOGIN` | `unkin-agent` | agent identity: selects `gitea/creds/<login>`; login whose comments watchpr ignores; agentws git identity |
+| `GITEA_CREDS_PATH` | `gitea/creds/$AGENT_LOGIN` | Vault path minting the Gitea token (wins over `AGENT_LOGIN`) |
 | `AGENTWS_SRC_ROOT` | `~/src/prodenv` | agentws source-of-truth checkout root |
 | `AGENTWS_ROOT` | `~/.cache/agentws` | agentws worktree root |
 | `AGENTWS_OWNER` | `unkin` | Gitea org that owns agentws-managed repos |
@@ -77,7 +82,7 @@ Config via env (all have defaults):
 Gitea tokens are ~1h ephemeral, so `agentws` never bakes one into a remote URL
 or config. `agentws token` prints a fresh token; `agentws credential get`
 implements the git credential protocol (reads the key=value request on stdin,
-and for the configured Gitea host only emits `username=unkin-agent` +
+and for the configured Gitea host only emits `username=$AGENT_LOGIN` +
 `password=<fresh token>`). `agentws new` wires this per worktree — it enables
 `extensions.worktreeConfig` on the repo once, then writes `user.name`,
 `user.email` and `credential.helper = !<agentws> credential` to the