github: Rework GraphQL batching and error handling to salvage partial work

[jerry-skydio]

Restructure the GitHub GraphQL layer around a flat, resizable query so that
partial results are never thrown away and failed work is re-transacted
precisely.

GraphqlQuery now holds a heterogeneous list of aliased fields (SingleQuery)
with stable per-prefix alias indices, so it can split() in half or subset()
to arbitrary fields without result aliases colliding. GithubQuery subclasses
it with the repo-specific field builders and result parsers.

endpoint.graphql no longer raises on GraphQL field errors. It returns a
GraphqlResponse carrying partial data plus per-field errors, and only raises
for a request error (200 with no data) or a non-retryable HTTP status. Retry
backoff is now driven by GitHub's headers, honoring Retry-After and waiting
out an exhausted rate-limit budget, and retries secondary-limit (403) as well
as transient 5xx.

Also recover a PR's id via a head-ref lookup when a create comes back
"already exists", so a later update never targets an empty id, and log the
remaining rate-limit budget at the end of a verbose run.

[jerry-skydio]

Reviews in this chain:
└#253 github: Rework GraphQL batching and error handling to salvage partial work
　└#285 build: Rename types to core_types to avoid stdlib shadow

[jerry-skydio]

#	head	base	diff	date	summary
0	978add51	4599c0ba	diff	May 20 17:00 PM	3 files changed, 892 insertions(+), 472 deletions(-)
1	550ecf7b	4599c0ba	diff	May 21 13:44 PM	1 file changed, 274 deletions(-)
2	3466dcba	b7cbff59	rebase	Jun 19 10:47 AM	0 files changed
3	252ff615	aab90897	diff	Jun 19 12:49 PM	0 files changed
4	9645b7ff	d1589ef0	diff	Jun 19 13:17 PM	1 file changed, 31 insertions(+), 23 deletions(-)
5	262cc29f	d1589ef0	diff	Jun 19 13:24 PM	1 file changed, 6 insertions(+), 6 deletions(-)
6	07520e54	d8008423	rebase	Jun 26 10:38 AM	0 files changed
7	40b85aad	d8008423	diff	Jul 23 17:58 PM	2 files changed, 211 insertions(+), 291 deletions(-)
8	e365bc59	d8008423	diff	Jul 24 13:00 PM	4 files changed, 911 insertions(+), 105 deletions(-)
9	bb9d762f	d8008423	diff	Jul 24 17:57 PM	2 files changed, 223 insertions(+), 220 deletions(-)
10	f39a1362	d8008423	diff	Jul 24 17:59 PM	1 file changed, 22 insertions(+), 26 deletions(-)
11	381121c3	d8008423	diff	Jul 24 18:19 PM	1 file changed, 38 insertions(+), 17 deletions(-)
12	1a44d102	d8008423	diff	Jul 24 18:31 PM	2 files changed, 49 insertions(+), 53 deletions(-)
13	65c7b87d	d8008423	diff	Jul 24 20:00 PM	6 files changed, 650 insertions(+), 355 deletions(-)
14	c559e5af	d8008423	diff	Jul 27 13:13 PM	0 files changed
15	5efbae68	bb92346f	rebase	Jul 27 13:47 PM	0 files changed
16	26f03961	74e57423	diff	Jul 27 14:34 PM	2 files changed, 181 insertions(+), 5 deletions(-)
17	6f986972	74e57423	diff	Jul 27 16:09 PM	1 file changed, 3 deletions(-)
18	d050ee57	74e57423	diff	Jul 27 16:12 PM	3 files changed, 16 insertions(+), 16 deletions(-)
19	483b9ab0	74e57423	diff	Jul 27 16:16 PM	2 files changed, 5 insertions(+), 7 deletions(-)
20	dff3aa7e	74e57423	diff	Jul 27 16:24 PM	2 files changed, 20 insertions(+), 7 deletions(-)