structpath: build computed paths and patterns from parts; lint rule

[denik]

Changes

• structpath.NewPattern(prev, parts...): the pattern counterpart of NewPath, with structpath.AnyKey (.*) and structpath.AnyIndex ([*]) wildcards.
• Computed paths and patterns in production code are built from parts instead of concatenating strings and parsing them (38 sites: library/path visitors, sync, permissions, validators).
• New ruleguard rule NoComputedStructpathParse: outside tests, MustParsePath/MustParsePattern/MustParsePaths take string literals only, and ParsePath/ParsePattern must not parse a formatted or concatenated string.
• Agent guideline in .agents/rules/style-guide-go.md.

Why

Parsing a computed string re-validates what the code just built, allocates, and panics on keys with characters the parser reserves. Literals, tests and user input still parse.

Stacked on #6954. No behaviour change: rendered paths and patterns are identical.

This pull request and its description were written by Isaac.

[eng-dev-ecosystem-bot]

Integration test report

Commit: f8f05cc

Run: 37924012388

	Env	✅​pass	🙈​skip	Time
✅​	aws linux-2core-8gb	276	17	4:12
✅​	aws-windows-latest-4core-16gb	278	15	3:56
✅​	azure linux-2core-8gb	275	17	4:11
✅​	azure-windows-latest-4core-16gb	277	15	3:41
✅​	gcp linux-2core-8gb	276	17	4:13
✅​	gcp-windows-latest-4core-16gb	278	15	3:49

Top 4 slowest tests (at least 2 minutes):

duration	env	testname
3:54	aws-windows-latest-4core-16gb	TestAccept
3:47	gcp-windows-latest-4core-16gb	TestAccept
3:39	azure-windows-latest-4core-16gb	TestAccept
2:01	gcp linux-2core-8gb	TestAccept