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: c0e686b

Run: 37746213972

	Env	✅​pass	🙈​skip	Time
✅​	aws linux	276	18	6:13
✅​	aws windows	278	16	4:19
✅​	azure linux	275	18	6:16
✅​	azure windows	277	16	4:34
✅​	gcp linux	276	18	6:23
✅​	gcp windows	278	16	4:09

Top 6 slowest tests (at least 2 minutes):

duration	env	testname
3:56	gcp windows	TestAccept
3:55	aws linux	TestAccept
3:52	gcp linux	TestAccept
3:44	aws windows	TestAccept
3:44	azure linux	TestAccept
3:37	azure windows	TestAccept