fix(session): bind workspace-verified repository scope at first bind (#714)

The independent review reproduced a real hole: the production first-bind path
never pinned repository/org, so the drift checks it feeds were skipped.

Root cause
----------
gitea_whoami, gitea_get_runtime_context, gitea_resolve_task_capability and
gitea_activate_profile all seeded the session context without repository or
org. First-bind-wins then made the mutation gate's later seed a no-op, and
assess_session_context only compares repository/org when the bound fields are
non-null. Result, reproduced in production order with the real REMOTES:

  after whoami:               repository=None org=None
  same-host repo=Other-Tools  -> NOT BLOCKED
  same-host org=Other-Org     -> NOT BLOCKED
  cross-host                  -> blocked (this part always worked)

Binding REMOTES defaults would not fix it: REMOTES['prgs'].repo is 'Timesheet'
(a default *target*, not an authorization scope, see #530), so pinning it fails
closed on every legitimate Gitea-Tools mutation. There was no trusted source
for repository scope, so this adds one.

Design
------
- New optional profile field `allowed_repositories`: canonical owner/repository
  slugs. An authorization boundary, never the binding itself. Config-only —
  no env var can widen or forge it.
- The session repository is derived from the verified, workspace-aligned git
  remote, never from caller-supplied org/repo, and validated against that
  allowlist. The session binds immutably to exactly one canonical slug; org is
  derived from it. A profile authorizing several repositories still binds to
  the single verified one.
- Every first-bind entry point now pins the same complete context. Activation
  rejects an unauthorized/unverifiable workspace. Mutations fail closed when
  repository/org is unverified, and a tool-level org/repo override that
  disagrees with the binding is rejected before the write.
- A mutation request can no longer establish, complete, or replace the binding
  (it previously seeded from `org or REMOTES[...]`, i.e. caller-controlled).
- Enforcement is opt-in per profile: a profile without the field keeps prior
  behaviour, so existing static-profile namespaces stay functional.

First-bind-wins, immutability, the RLock, profile isolation, host/identity
validation and the private pytest-only reset are unchanged.

gitea_auth.get_profile() built an explicit whitelist dict and silently dropped
`allowed_repositories`; the new integration tests caught that the scope was
never enforced through the real path.

Tests
-----
New tests/test_issue_714_production_first_bind.py drives the real entry points
in production order rather than constructing _SessionContext directly. Covers
complete first bind via each entry point, same-host repo/org drift, Timesheet
and other-repo/other-org rejection, unverified and unauthorized workspaces,
caller values unable to establish the binding, concurrent init selecting one
repository, and the PR #715 author path staying functional. Mutation-verified:
disabling trusted derivation, override validation, the scope check, or the
get_profile passthrough each fails these tests (9/8/4/5 failures).

No security assertion was weakened, removed, skipped, or rewritten.

Focused: 26 passed. Issue #714 total: 46 passed. Prior failing modules: 240
passed. Config/profile/remote modules: 130 passed. Full suite: 2739 passed,
6 skipped, 1 pre-existing warning, 161 subtests in 25.80s.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

gitea_auth.py

@@ -570,6 +570,10 @@ def get_profile():
         "profile_name": name,
         "allowed_operations": ops,
         "forbidden_operations": forbidden,
+        # #714 repository authorization boundary. Config-only on purpose: an
+        # environment variable must never widen or forge the set of
+        # repositories a session may bind to.
+        "allowed_repositories": _json_list("allowed_repositories"),
         "audit_label": audit_label,
         "token_source_name": token_source,
         "auth_source_type": auth_type,