fix: parse inline --flag=value form in claude_args

[HumphreySun98]

What

claude_args written in the inline --flag=value form — which the Claude Code CLI itself accepts — is silently mis-parsed: the flag name and value are never split, so the whole token becomes a bogus extraArgs key and the value is dropped.

Why it matters

parseClaudeArgsToExtraArgs (base-action/src/parse-sdk-options.ts) does const flag = arg.slice(2) and never splits on =. So --allowedTools=Edit,Read becomes the key "allowedTools=Edit,Read" (value null) instead of allowedTools → "Edit,Read". Downstream, parseSdkOptions keys its interception/merge/detection off exact flag names, so with the = form:

• --allowedTools= / --disallowedTools= / --add-dir= are not merged into sdkOptions — the tools/dirs are silently dropped, and the raw allowedTools=… token leaks through to the CLI as a malformed flag (reintroducing the duplicate---allowedTools clobbering that --allowed-tools parsing broken after USE_AGENT_SDK default changed to true (PR #738) #746 hardened against).
• --json-schema={…} is not detected — hasJsonSchema stays false, so structured output is silently skipped.

Reproduced on current main:

--allowedTools Edit,Read   → allowedTools: ["Edit","Read"]                              (space form, works)
--allowedTools=Edit,Read   → allowedTools: undefined, extraArgs: {"allowedTools=Edit,Read": null}   (BUG)
--json-schema={...}        → hasJsonSchema: false                                       (BUG)

Fix

In the arg loop, split the flag on the first = only (so a value that itself contains =, e.g. inline JSON, is preserved) and route the inline value through the same accumulating / non-accumulating handling used for the space-separated form. --flag=value and --flag value now produce identical results, and --flag=value consumes no following token (matching CLI semantics).

Tests

Added a --flag=value (inline value) form suite to base-action/test/parse-sdk-options.test.ts: the =-form allowedTools equals the space form; --json-schema=… sets hasJsonSchema; split-on-first-= preserves = inside the value; disallowedTools / add-dir work; and repeated --allowedTools=… accumulate. Each fails before the fix and passes after.

Verification

• bun test → 774 pass, 0 fail (5 new).
• bun run typecheck → clean.
• bun run format:check → clean.