fix(github): don't synthesize branch_name, defer to workspace fallback

[corylanou]

Fixes #95.

Context

The GitHub adapter (elixir/lib/symphony_elixir/github/client.ex:681) was synthesizing branch_name: derive_branch_name(number, title) for every issue, producing values like issue-95-regression-workspace-.... This silently overrode the operator's hooks.after_create branch (symphony/<id>) and broke the Symphony Isolation Contract self-check, causing agents to self-block immediately. Confirmed today on issues #93 and #95 — both required manual git branch -m interventions to unblock.

TL;DR

Stop the GitHub adapter from imposing a synthesized branch name on the workspace. Defer to workspace.ex's existing fallback (symphony/<id>) so operator hooks aren't overridden.

Summary

• github/client.ex: change branch_name: derive_branch_name(number, title) to branch_name: nil and remove the now-unused derive_branch_name/2 definition.
• Workspace flow (workspace.ex:423-427): unchanged. When branch_name is nil, target_branch_name/1 falls through to fallback_branch_name(issue_identifier) → "symphony/#{...}", which matches what operators configure in hooks.after_create.

Alternatives

• Make the synthesized pattern configurable (e.g. tracker.branch_name_pattern) — rejected; adds YAML surface without a concrete need beyond what hooks.after_create already covers.
• Detect the hook's branch in workspace.ex and prefer it — rejected; requires workspace.ex to be aware of hook side-effects, leaking abstractions.

Test Plan

• make -C elixir all — 666 tests, 0 failures from this change (one pre-existing flake in OrchestratorBudgetTest that passes in isolation, unrelated).
• All 204 github/ adapter tests still pass.
• No tests asserted the old synthesized branch name, so no regressions.
• Manual verification: re-dispatching regression(workspace): new agent runs bypass hooks.after_create, branch named from issue title slug instead of symphony/<id> #95 (or any issue) after this lands should produce a worktree on symphony/<id> per the hook, not issue-N-.... Agent isolation check passes without manual branch rename.

[corylanou]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Delightful!

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